Revert "[FSDP] Option to keep grads in lower prec (#85134)"
This reverts commit 607eccb13ca586f775fb09daeb728a4b4e30ebdd.
Reverted https://github.com/pytorch/pytorch/pull/85134 on behalf of https://github.com/ZainRizvi due to broke trunk, failing the tests test_grads_reduced_precision (main.TestFSDPMixedPrecisionUnsharded)
