标题：算子gather_tree存在安全问题/bug
版本、环境信息：
   1）PaddlePaddle版本：2.1.0
   2）CPU/GPU：CPU
   3）系统环境: ubuntu 20.04
   4）Python版本号: 3.8.5

PoC：

import paddle
import paddle.fluid as fluid
import numpy as  np

ids = paddle.to_tensor([[2,2],[6,1]])
parents = paddle.to_tensor([[2,2],[6,1]])
out = paddle.nn.functional.gather_tree(ids,parents)

崩溃信息

--------------------------------------
C++ Traceback (most recent call last):
--------------------------------------
0   paddle::imperative::Tracer::TraceOp(std::string const&, paddle::imperative::NameVarBaseMap const&, paddle::imperative::NameVarBaseMap const&, paddle::framework::AttributeMap, std::map<std::string, std::string, std::less<std::string >, std::allocator<std::pair<std::string const, std::string > > > const&)
1   paddle::imperative::Tracer::TraceOp(std::string const&, paddle::imperative::NameVarBaseMap const&, paddle::imperative::NameVarBaseMap const&, paddle::framework::AttributeMap, paddle::platform::Place const&, bool, std::map<std::string, std::string, std::less<std::string >, std::allocator<std::pair<std::string const, std::string > > > const&)
2   paddle::imperative::PreparedOp::Run(paddle::imperative::NameVarBaseMap const&, paddle::imperative::NameVarBaseMap const&, paddle::framework::AttributeMap const&)
3   std::_Function_handler<void (paddle::framework::ExecutionContext const&), paddle::framework::OpKernelRegistrarFunctor<paddle::platform::CPUPlace, false, 1ul, paddle::operators::GatherTreeOpKernel<int>, paddle::operators::GatherTreeOpKernel<long> >::operator()(char const*, char const*, int) const::{lambda(paddle::framework::ExecutionContext const&)#1}>::_M_invoke(std::_Any_data const&, paddle::framework::ExecutionContext const&)
4   paddle::framework::SignalHandle(char const*, int)
5   paddle::platform::GetCurrentTraceBackString[abi:cxx11]()

----------------------
Error Message Summary:
----------------------
FatalError: `Segmentation fault` is detected by the operating system.
  [TimeInfo:***Aborted at 1623049987 (unix time) try "date -d @1623049987" if you are using GNU date***]
  [SignalInfo:***SIGSEGV (@0x7ff184021000) received by PID 4597 (TID 0x7ff197dad740) from PID 18446744071629312000***]

Segmentation fault (core dumped)

分析:

部分源码: (https://github.com/PaddlePaddle/Paddle/blob/v2.1.0/paddle/fluid/operators/gather_tree_op.h#L45)

auto &ids_dims = ids->dims();
auto max_length = ids_dims[0];
auto batch_size = ids_dims[1];
auto beam_size = ids_dims[2];         //[1]

for (int batch = 0; batch < batch_size; batch++) {
    for (int beam = 0; beam < beam_size; beam++) {
        auto idx = (max_length - 1) * batch_size * beam_size +
                   batch * beam_size + beam;      //[2]
        out_data[idx] = ids_data[idx];             //[3]

对ids的维度未进行检查，当ids的shape设置不当时（维度小于3，例设为2），导致
问题① [1]处访问未初始化内存;
问题② 当该内存处值过大时，beam_size值过大，从而导致[2]中idx值过大，从而[3]处发生越界访问的问题，导致程序崩溃。崩溃现场部分变量值如下：

(gdb) i locals
idx = 0x11ee400000110
ids_dims = @0x1f72ec8: {
  static kMaxRank = 0x9,
  ids_dims = @0x1e48f78: {
  static kMaxRank = 0x9,
  dim_ = {
    <paddle::framework::Array<long, 9>> = {
      static kSize = <optimized out>,
      data_ = {0x2, 0x2, 0x8f7200000088, 0x9272000000907200, 0x9f72000000, 0x33720000003372, 0x7200000033720000, 0x6d3c08da00000034, 0x1e3e656c75646f}
    },
  },
  rank_ = 0x2
}
beam_size = 0x8f7200000088

debug信息：

[ Legend: Modified register | Code | Heap | Stack | String ]
────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── registers ────
$rax   : 0x9771e7306e8c0
$rbx   : 0x00007fffffffd3e8  →  0x00007ffea33247aa  →  "gather_tree"
$rcx   : 0x8f72000000880
$rdx   : 0x9771e730708c0
$rsp   : 0x00007fffffffcee0  →  0x00007fffffffd090  →  0x00007ffea5feed70  →  0x00007ffea23853d0  →  <paddle::imperative::DygraphExecutionContext<paddle::imperative::VarBase>::~DygraphExecutionContext()+0> endbr64
$rbp   : 0x00007fffffffcfa0  →  0x00007fffffffcfe0  →  0x00007fffffffd010  →  0x00007fffffffd040  →  0x00007fffffffd210  →  0x00007fffffffd250  →  0x00007fffffffd440  →  0x00007fffffffd480
$rsi   : 0x2
$rdi   : 0x0000000001e48f78  →  0x0000000000000002
$rip   : 0x00007ffe9fc8afe7  →  <paddle::operators::GatherTreeOpKernel<long>::Compute(paddle::framework::ExecutionContext+0> mov rax, QWORD PTR [rax]
$r8    : 0x1
$r9    : 0x0
$r10   : 0xc70f6907
$r11   : 0x00007ffff7fbabe0  →  0x0000000001f9ba50  →  0x00007ffff79cbb70  →  0x0000000000000012
$r12   : 0x0
$r13   : 0x00007fffffffd980  →  0x00007fffffffd990  →  "gather_tree"
$r14   : 0x2
$r15   : 0x0
$eflags: [zero carry PARITY adjust sign trap INTERRUPT direction overflow RESUME virtualx86 identification]
$cs: 0x0033 $ss: 0x002b $ds: 0x0000 $es: 0x0000 $fs: 0x0000 $gs: 0x0000
────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── stack ────
0x00007fffffffcee0│+0x0000: 0x00007fffffffd090  →  0x00007ffea5feed70  →  0x00007ffea23853d0  →  <paddle::imperative::DygraphExecutionContext<paddle::imperative::VarBase>::~DygraphExecutionContext()+0> endbr64          ← $rsp
0x00007fffffffcee8│+0x0008: 0x00007fffffffcfc0  →  0x00007ffea5fb1d10  →  0x00007ffe9fc8ad4c  →  <paddle::operators::GatherTreeOpKernel<long>::Compute(paddle::framework::ExecutionContext+0> endbr64
0x00007fffffffcef0│+0x0010: 0x00000000ffffcf20
0x00007fffffffcef8│+0x0018: 0x00007ffe00000000
0x00007fffffffcf00│+0x0020: 0x0000000000000000
0x00007fffffffcf08│+0x0028: 0x0000000001e48f58  →  0x4c055a5f00000000
0x00007fffffffcf10│+0x0030: 0x0000000001e3d838  →  0x7369206500000000
0x00007fffffffcf18│+0x0038: 0x0000000001d07548  →  0x0000000000000000
──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── code:x86:64 ────
   0x7ffe9fc8afd8 <paddle::operators::GatherTreeOpKernel<long>::Compute(paddle::framework::ExecutionContext+0> lea    rcx, [rdx*8+0x0]
   0x7ffe9fc8afe0 <paddle::operators::GatherTreeOpKernel<long>::Compute(paddle::framework::ExecutionContext+0> mov    rdx, QWORD PTR [rbp-0x70]
   0x7ffe9fc8afe4 <paddle::operators::GatherTreeOpKernel<long>::Compute(paddle::framework::ExecutionContext+0> add    rdx, rcx
 → 0x7ffe9fc8afe7 <paddle::operators::GatherTreeOpKernel<long>::Compute(paddle::framework::ExecutionContext+0> mov    rax, QWORD PTR [rax]
   0x7ffe9fc8afea <paddle::operators::GatherTreeOpKernel<long>::Compute(paddle::framework::ExecutionContext+0> mov    QWORD PTR [rdx], rax
   0x7ffe9fc8afed <paddle::operators::GatherTreeOpKernel<long>::Compute(paddle::framework::ExecutionContext+0> mov    rax, QWORD PTR [rbp-0x48]
   0x7ffe9fc8aff1 <paddle::operators::GatherTreeOpKernel<long>::Compute(paddle::framework::ExecutionContext+0> lea    rdx, [rax*8+0x0]
   0x7ffe9fc8aff9 <paddle::operators::GatherTreeOpKernel<long>::Compute(paddle::framework::ExecutionContext+0> mov    rax, QWORD PTR [rbp-0x78]
   0x7ffe9fc8affd <paddle::operators::GatherTreeOpKernel<long>::Compute(paddle::framework::ExecutionContext+0> add    rax, rdx
──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── threads ────
[#0] Id 1, Name: "python", stopped 0x7ffe9fc8afe7 in paddle::operators::GatherTreeOpKernel<long>::Compute (), reason: SIGSEGV
────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── trace ────
[#0] 0x7ffe9fc8afe7 → paddle::operators::GatherTreeOpKernel<long>::Compute(this=0x7fffffffcfc0, ctx=@0x7fffffffd090)
[#1] 0x7ffe9fc8a173 → paddle::framework::OpKernelRegistrarFunctor<paddle::platform::CPUPlace, false, 1ul, paddle::operators::GatherTreeOpKernel<int>, paddle::operators::GatherTreeOpKernel<long> >::operator()(char const*, char const*, int) const::{lambda(paddle::framework::ExecutionContext const&)#1}::operator()(paddle::framework::ExecutionContext const&) const(this=0x7fffffffd3e8, ctx=@0x7fffffffd090)
[#2] 0x7ffe9fc8c047 → std::_Function_handler<void (paddle::framework::ExecutionContext const&), paddle::framework::OpKernelRegistrarFunctor<paddle::platform::CPUPlace, false, 1ul, paddle::operators::GatherTreeOpKernel<int>, paddle::operators::GatherTreeOpKernel<long> >::operator()(char const*, char const*, int) const::{lambda(paddle::framework::ExecutionContext const&)#1}>::_M_invoke(std::_Any_data const&, paddle::framework::ExecutionContext const&)(__functor=@0x7fffffffd3e8, __args#0=@0x7fffffffd090)
[#3] 0x7ffea2386ee3 → std::function<void (paddle::framework::ExecutionContext const&)>::operator()(paddle::framework::ExecutionContext const&) const(this=0x7fffffffd3e8, __args#0=@0x7fffffffd090)

(gdb) x/10x $rax
0x9771e7306e8c0:        Cannot access memory at address 0x9771e7306e8c0