[FSDP] Fix the FQN not found issue for load sharded_state_dict when using activation checkpoint (#84253)
The current sharded_state_dict load will fail if activation checkpoint is also enabled. This PR fixes the issue.
Differential Revision: [D39125431](https://our.internmc.facebook.com/intern/diff/D39125431/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84253
Approved by: https://github.com/awgu
