[doc][c10d] fixup fsdp tutorial

[c-p-i-o]

Summary: Fix up the FSDP tutorial to get it functional again.

1. Add missing import for load_dataset.
2. Use checkpoint instead of _shard.checkpoint to get rid of a warning.
3. Add nlp to requirements.txt
4. Get rid of load_metric as this function does not exist in new datasets module.
5. Add legacy=False to get rid of tokenizer warnings.

Test Plan: Ran the tutorial as follows and ensured that it ran successfully:

torchrun --nnodes=1 --nproc_per_node=2 T5_training.py
W1031 09:46:49.166000 2847649 torch/distributed/run.py:793]
W1031 09:46:49.166000 2847649 torch/distributed/run.py:793]
*****************************************
W1031 09:46:49.166000 2847649 torch/distributed/run.py:793] Setting
OMP_NUM_THREADS environment variable for each process to be 1 in
default, to avoid your system being overloaded, please further tune the
variable for optimal performance in your application as needed.
W1031 09:46:49.166000 2847649 torch/distributed/run.py:793]
*****************************************
dict_keys(['train', 'validation', 'test'])
Size of train dataset:  (157252, 3)
Size of Validation dataset:  (5599, 3)
dict_keys(['train', 'validation', 'test'])
Size of train dataset:  (157252, 3)
Size of Validation dataset:  (5599, 3)
bFloat16 enabled for mixed precision - using bfSixteen policy

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[fduwjj]

looks like running python example failed?

[c-p-i-o]

looks like running python example failed?

Unrelated to my change - but I fixed it anyway. Needed to update to a newer Python version in CI.

See the additional diff I made to .github/workflows/main_python.yml. Let me know if you want me to split this out into a separate change.

[c-p-i-o]

@fduwjj - CI is green now. As mentioned, the CI broke because of some dependency changes upstream and I had to do 3 things to fix Run Python Examples.

1. Use newer Python in .github/workflows
2. Pin numpy to below version 2.
3. Pin torchvision.

[c-p-i-o]

This change will be rebased on https://github.com/pytorch/examples/pull/1299 to fix the failing Python Examples.