Fix to-fsdp2: drop REMOVED / NOT_YET_IMPLEMENTED FSDP1 keys instead of leaking them

[lollinng]

Problem

accelerate to-fsdp2 converts an FSDP1 config to FSDP2. In convert_config_to_fsdp2, the first branch of the per-key loop was:

conversion_status = ARGUMENT_KEY_MAPPING.get(key, None)
if isinstance(conversion_status, ConversionStatus) or conversion_status is None:
    conversion_status = key
    new_fsdp_config[conversion_status] = value
    continue

This carries the key over and continues for both "key not in the mapping" (None) and the ConversionStatus enum values (REMOVED / NOT_YET_IMPLEMENTED). As a result, the REMOVED / NOT_YET_IMPLEMENTED handling just below was dead code, and FSDP1-only keys — fsdp_backward_prefetch, fsdp_sync_module_states, fsdp_use_orig_params (REMOVED) and fsdp_forward_prefetch (NOT_YET_IMPLEMENTED) — leaked into the converted FSDP2 config instead of being dropped.

Fix

Only carry over keys that are genuinely absent from the mapping (conversion_status is None); let ConversionStatus values fall through to the REMOVED / NOT_YET_IMPLEMENTED branches that drop them.

Testing done (CPU)

Added tests/test_cli.py::ToFSDP2Tester::test_convert_config_drops_removed_and_unimplemented_keys:

1 passed

It asserts the four FSDP1-only keys are dropped, that a renamed+value-mapped key (fsdp_sharding_strategy: FULL_SHARD → fsdp_reshard_after_forward: True) and pass-through / unknown keys are preserved, and that fsdp_version is set to 2. ruff clean.