Add nested tensor contiguous (#82147)
### Description
<!-- What did you change and why was it needed? -->
The nested_tensor impl for `contiguous` was currently disabled. Prior to the work on nested_tensor transpose. Only contiguous nested tensors could be created from python. However now is possible to create nested tensors that are non contiguous. This pr links up the existing function used at the c++ level to the python function.
### Tests
Updated Test in `test/test_nestedtensor.py`
### Notes
The inference mode had to be removed for this test. This is because the func `.contiguous` is a composite implicit function. Currently this does not work in inference mode. However: https://github.com/pytorch/pytorch/pull/81838 should fix that issue.
### Why
When writing kernels in Triton for nested tensors I exposed a helper function that returned the "Buffer" tensor to python. Now contiguity can be checked before running any triton kernel. Also a good follow up would be making `nt.contiguous` on non contiguous nested tensors return a contiguous nested tensor.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82147
Approved by: https://github.com/jbschlosser
