Fix ZeRO legacy grad-hook crash when next_functions is missing

[harshang03]

Summary

• harden legacy (torch<2.1) grad-hook registration when checkpointed graphs do not expose param.expand_as(...).grad_fn.next_functions
• fall back to param.register_hook safely instead of crashing with 'NoneType' object has no attribute next_functions'
• wire hook-handle registration defensively in ZeRO-1/2, ZeRO-3, and BF16 optimizer paths and add focused regression tests

Testing

• python3 -m pytest tests/unit/runtime/test_register_grad_hook.py (fails in this environment: missing dependency torch)

Fixes #7830