Support batching variable size tensors using nested tensors

[mosheraboh]

Is your feature request related to a problem? Please describe.
Support batching variable size tensors using nested tensors (https://pytorch.org/tutorials/prototype/nestedtensor.html)
To avoid padding and improve the running time.

Describe the solution you'd like
Add such an option in CollateDefault as an alternative to CollateDefault.pad_all_tensors_to_same_size.

Describe alternatives you've considered
N/A

Additional context
N/A

[SagiPolaczek]

Note that currently there is a user warning (torch 1.13.0):

UserWarning: The PyTorch API of nested tensors is in prototype stage and will change in the near future.

[YoelShoshan]

if/when it works well it could be a very nice GRAM saver

[SagiPolaczek]

For ref: General NestedTensor op coverage tracking issue.