Replace usage of nvFuser's broadcast_in_dim in executor with broadcast+expand ops

🚀 Feature

Replace usage of nvFuser's broadcast_in_dim in the executor with broadcast + expand operations.

Motivation

Currently, the Thunder executor uses nvFuser's broadcast_in_dim for broadcasting, as seen here.

Jacob suggested (in this PR review) that using explicit broadcast and expand ops would be preferable. This change would improve the developer experience by creating a closer match between Python-defined fusions and nvFuser's "math/kernel" representation.

Pitch

Update the executor logic to replace the direct usage of broadcast_in_dim with a combination of broadcast and expand operations where applicable.

Alternatives

Continue using broadcast_in_dim as it is currently implemented.

Additional context

• Current implementation: https://github.com/Lightning-AI/lightning-thunder/blob/9ca2b0d65e491f564dc905bfe80b1777f50daafb/thunder/executors/nvfuserex_impl.py#L1180-L1189
• Suggestion source: https://github.com/NVIDIA/Fuser/pull/5507#pullrequestreview-3478756789

cc @mruberry

Read thunder/executors/nvfuserex_impl.py around lines 1180-1189 and the referenced nvFuser review for context. Trace the executor’s current broadcast_in_dim path and replace it with broadcast plus expand where applicable; done means the executor uses the explicit operations and better matches the Python-defined fusion representation.