[FSDP] Validate buffer dtype in pre-forward hook for FSDP mixed precision tests (#106231)
Summary: https://github.com/pytorch/pytorch/issues/104740
Test Plan: buck2 test mode/dev-nosan caffe2/test/distributed/fsdp:fsdp_mixed_precision -- test_full_precision_in_eval_buffers
Reviewed By: awgu
Differential Revision: D47858769
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106231
Approved by: https://github.com/awgu
