Pre-overlap collective bucketing pass for FSDP/DDP

[fmassa]

PyTorch's overlap scheduler adds sequential timeline dependencies between all consecutive events on each process group, which prevents its downstream bucketer from merging collectives that were originally independent. This PR adds a pre-pass that merges per-parameter FSDP/DDP collectives before the overlap scheduler runs, so it sees fewer, larger collectives.

The pass targets three collective types: forward all-gathers (param-derived), backward reduce-scatters (terminal-derived), and backward all-reduces (terminal-derived, for DDP).

The implementation is split into two phases:

• Tagging runs on the joint graph (where placeholder metadata is available) and marks eligible collectives via node.meta. Tags survive the fw/bw partition via node_copy's shallow copy.
• Bucketing runs on the split fw/bw graphs inside the compiler, reads the tags, and merges collectives using PyTorch's existing merge functions.

This PR also fixes a pre-existing bug in _copy_descriptors_and_rename_placeholders where make_fx could nest the output tuple while desc stayed flat, causing get_all_input_and_grad_nodes's zip to silently mismatch output nodes with their descriptors.

Results on LLaMA: AG 290→98 (fwd), 225→97 (bwd recomputed), RS 290→194 (bwd).

Test plan

• example_autoparallel.py passes (compile=False)
• example_llama3.py passes
• pytest tests/ passes

Authored with Claude.