[BUG] - Depracate the usage of Nested Tensors

[svekars]

Add Link

intermediate_source/transformer_building_blocks.py
unstable_source/nestedtensor.py
pytorch/pytorch#171814

Describe the bug

intermediate_source/transformer_building_blocks.py

We need to depracate the usage of NestedTensors in all tutorials and replace with varlen attention.
We put a warning to not use them but we should provide what should be used instead.

Describe your environment

• PyTorch 2.10

cc @albanD @jbschlosser