main PR: #4423

What does this PR do ?

split_swiglu_linear_fc1 and split_gdn_fused originally used data.numel() // tp_mesh.mesh.numel() to compute component slices (w_slice/v_slice for SWiGLU, comp_slice for GDN), then intersected them with fsdp_slice to determine each rank's portion. When data is a DTensor, data.numel() returns the global element count (=param.numel()), so component slices and fsdp_slice are both in item coordinates and the intersection is valid. However, when data is a plain Tensor (optimizer states created by FusedAdam), data.numel() returns the local shard size on this rank, making the component slices's range far smaller than the item coordinate space and resulting in an intersection across incompatible coordinate systems.

The fix replaces data.numel() with dist_param.numel() (dist_param is always a DTensor and therefore reflects the global element count), ensuring fsdp_slice and all component slices are in item coordinates.

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