Revert D35000703: [WIP][FSDP] Mixed precision enablement
Test Plan: revert-hammer
Differential Revision:
D35000703 (https://github.com/pytorch/pytorch/commit/6b0b088c6caef54d35ad04defe07a4a94831dd27)
Original commit changeset: 4bd7937ff36b
Original Phabricator Diff: D35000703 (https://github.com/pytorch/pytorch/commit/6b0b088c6caef54d35ad04defe07a4a94831dd27)
fbshipit-source-id: 99295ea4ff022dea22b89d9d965ea4261cdf8826
(cherry picked from commit 05ed48197d652a911cdd040aea0fc67768ef10e5)
