Add free storage API for FSDP

[Seventeen17]

Fix #263

When using fsdp with Dit or T5, directly deleting the model with del model does not release the GPU memory occupied by the model.

This is because the memory allocated by FSDP's flat_param requires manual free.

To solve this, an additional free_model interface is added to distributed.fsdp.

This interface allows users to manually release the GPU memory occupied by the model sharded by FSDP in scenarios where different model checkpoints need to be switched.

Example Usage:

diff --git a/generate.py b/generate.py
index 1b1a9d7..307c949 100644
--- a/generate.py
+++ b/generate.py
@@ -321,6 +321,9 @@ def generate(args):
             seed=args.base_seed,
             offload_model=args.offload_model)
 
+        from wan.distributed.fsdp import free_model
+        free_model(wan_t2v.model)
+
     else:
         if args.prompt is None:
             args.prompt = EXAMPLE_PROMPT[args.task]["prompt"]