[FSDP()][23/N] Refactor handle attr initialization (#87938)
**`_init_param_attributes()` -> `init_flat_param_attributes()`**
We move `_init_param_attributes()` to `FlatParamHandle.init_flat_param_attributes()` (as already marked as to-do during previous refactoring).
**`_reset_lazy_init()`**
We no longer delete `_local_shard` from each `FlatParameter` in `_reset_lazy_init()`.
**Analysis**
Thus, the two semantic differences are that we remove the initial `if hasattr(p, "_local_shard")` early return in `_init_param_attributes()` and the `delattr(p, "_local_shard")` in `_reset_lazy_init()`.
This is safe because
- If we never call `_reset_lazy_init()`, then `init_flat_param_attributes()` is only called once. There is no opportunity for an early return.
- If we call `_reset_lazy_init()`, then `init_flat_param_attributes()` will be called again in the next `_lazy_init()`. However, since we removed the early return, all of the attributes initialized in `init_flat_param_attributes()` simply get re-initialized and override any existing attributes.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87938
Approved by: https://github.com/mrshenli
