我已经导入Vit-b32模型并对其进行微调，以执行回波图像分类任务。现在我想可视化注意力图，这样我就可以知道模型在执行分类任务时关注的是图像的哪一部分。但是我无法做到这一点，当我试图在微调模型后可视化注意力Map时，我得到了一个错误。下面是代码：

!pip install --quiet vit-keras
from vit_keras import vit
vit_model = vit.vit_b32(
        image_size = IMAGE_SIZE,
        activation = 'softmax',
        pretrained = True,
        include_top = False,
        pretrained_top = False,
        classes = 3)

当我尝试在没有任何微调的情况下可视化注意力Map时，它正在工作，没有任何错误：

from vit_keras import visualize

    x = test_gen.next()
    image = x[0]
    
    attention_map = visualize.attention_map(model = vit_model, image = image)
    
    # Plot results
    fig, (ax1, ax2) = plt.subplots(ncols = 2)
    ax1.axis('off')
    ax2.axis('off')
    ax1.set_title('Original')
    ax2.set_title('Attention Map')
    _ = ax1.imshow(image)
    _ = ax2.imshow(attention_map)

现在，在下面的代码中，我已经向模型添加了一些分类层并对其进行了微调：

model = tf.keras.Sequential([
        vit_model,
        tf.keras.layers.Flatten(),
        tf.keras.layers.BatchNormalization(),
        tf.keras.layers.Dense(11, activation = tfa.activations.gelu),
        tf.keras.layers.BatchNormalization(),
        tf.keras.layers.Dense(3, 'softmax')
    ],
    name = 'vision_transformer')

model.summary()

下面是上述单元格的输出：

> Model: "vision_transformer"
> _________________________________________________________________ Layer (type)                 Output Shape              Param #   
> ================================================================= vit-b32 (Functional)         (None, 768)               87455232  
> _________________________________________________________________ flatten_1 (Flatten)          (None, 768)               0         
> _________________________________________________________________ batch_normalization_2 (Batch (None, 768)               3072      
> _________________________________________________________________ dense_2 (Dense)              (None, 11)                8459      
> _________________________________________________________________ batch_normalization_3 (Batch (None, 11)                44        
> _________________________________________________________________ dense_3 (Dense)              (None, 3)                 36        
> ================================================================= Total params: 87,466,843 Trainable params: 87,465,285 Non-trainable
> params: 1,558
> _________________________________________________________________

现在我已经在我自己的医疗数据集上训练了模型：

learning_rate = 1e-4

optimizer = tfa.optimizers.RectifiedAdam(learning_rate = learning_rate)

model.compile(optimizer = optimizer, 
              loss = tf.keras.losses.CategoricalCrossentropy(label_smoothing = 0.2), 
              metrics = ['accuracy'])

STEP_SIZE_TRAIN = train_gen.n // train_gen.batch_size
STEP_SIZE_VALID = valid_gen.n // valid_gen.batch_size

reduce_lr = tf.keras.callbacks.ReduceLROnPlateau(monitor = 'val_accuracy',
                                                 factor = 0.2,
                                                 patience = 2,
                                                 verbose = 1,
                                                 min_delta = 1e-4,
                                                 min_lr = 1e-6,
                                                 mode = 'max')

earlystopping = tf.keras.callbacks.EarlyStopping(monitor = 'val_accuracy',
                                                 min_delta = 1e-4,
                                                 patience = 5,
                                                 mode = 'max',
                                                 restore_best_weights = True,
                                                 verbose = 1)

checkpointer = tf.keras.callbacks.ModelCheckpoint(filepath = './model.hdf5',
                                                  monitor = 'val_accuracy', 
                                                  verbose = 1, 
                                                  save_best_only = True,
                                                  save_weights_only = True,
                                                  mode = 'max')

callbacks = [earlystopping, reduce_lr, checkpointer]

model.fit(x = train_gen,
          steps_per_epoch = STEP_SIZE_TRAIN,
          validation_data = valid_gen,
          validation_steps = STEP_SIZE_VALID,
          epochs = EPOCHS,
          callbacks = callbacks)

model.save('model.h5', save_weights_only = True)

训练后，当我试图可视化模型的注意力Map时，它显示错误：

from vit_keras import visualize

x = test_gen.next()
image = x[0]

attention_map = visualize.attention_map(model = model, image = image)

# Plot results
fig, (ax1, ax2) = plt.subplots(ncols = 2)
ax1.axis('off')
ax2.axis('off')
ax1.set_title('Original')
ax2.set_title('Attention Map')
_ = ax1.imshow(image)
_ = ax2.imshow(attention_map)

下面是以下错误：

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-13-f208f2d2b771> in <module>
      4 image = x[0]
      5 
----> 6 attention_map = visualize.attention_map(model = model, image = image)
      7 
      8 # Plot results

/opt/conda/lib/python3.7/site-packages/vit_keras/visualize.py in attention_map(model, image)
     14     """
     15     size = model.input_shape[1]
---> 16     grid_size = int(np.sqrt(model.layers[5].output_shape[0][-2] - 1))
     17 
     18     # Prepare the input

TypeError: 'NoneType' object is not subscriptable

请提出一些方法来纠正上述错误，并可视化微调模型的注意力Map