[LayerSkip] Early Exit Loss

[mostafaelhoushi]

Describe the solution you would like:

• Enable the training script to access outputs of intermediate layers
• Modify loss function to incorprate outputs of earlier layers

Describe the alternatives you have considered:
Different approaches to implement:

• store output of each layer in a dictionary variable of the model
• return outputs of intermediate layers as an additional return for the forward() function (the disadvantage of this is that it can introduce errors into existing training loops)
• Utilize existing hook mechanism

Additional Context:
This is to enable implementing ideas from various papers such as:

• Depth Adaptive Transformers
• LayerSkip