Reland Simplify handle indexing (#105006) (#106357)
This reverts commit a9a3c456495ccddff55e088ebf395c599db62d12.
This PR changes the following:
- `_ExecOrderData.handle_to_handle_index` -> `FlatParamHandle._handle_index`
- `_ExecOrderData.handles_to_pre_forward_order_index` -> `FlatParamHandle._pre_forward_order_index`
- `_ExecOrderData.handles_to_post_forward_order_index` -> `FlatParamHandle._post_forward_index`
- `_FSDPState._needs_pre_forward_unshard` -> `FlatParamHandle._needs_pre_forward_unshard`
- `_FSDPState._needs_pre_backward_unshard` -> `FlatParamHandle._needs_pre_backward_unshard`
- `_FSDPState._handles_prefetched` -> `FlatParamHandle._prefetched`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106357
Approved by: https://github.com/awgu
