Fix comment about get_fw_grad_mode() only being used in custom Function (#90790)
Addresses
https://github.com/pytorch/pytorch/pull/90240#issuecomment-1349596445
This was the only comment I found after grepping the codebase, but
please let me know if I missed others.
Test Plan:
- code reading
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90790
Approved by: https://github.com/soulitzer
