Guard vocab reduce_scatter on TP > 1

[mathemakitten]

What does this PR do ?

This cuts 32us per decode step when TP=1. reduce_from_tensor_model_parallel_region is a no-op but it still hits torch.autograd.Function, which does a non-trivial amount of host-side work despite the no-op (records input metadata, checks whether autograd needs to record this call, calls device guards, etc).

Verified empirically, also visible in profiling

Before:
9b | dynamic | graphs 12 | uvm 0 | requests: cli, n 1, g 10, dur 1.0e+01 r/sec 1.0e+02 | bf: 10 GB, 399 chunks [r 396, t 16384] … throughput: 40.344 tok/s … total time: 0.248s … mem 28.7/32.1 GB … steps: 10 … capture --

After:
9b | dynamic | graphs 12 | uvm 0 | requests: cli, n 1, g 10, dur 1.0e+01 r/sec 1.0e+02 | bf: 10 GB, 399 chunks [r 396, t 16384] … throughput: 57.025 tok/s … total time: 0.175s … mem 28.7/32.1 GB … steps: 10 … capture --

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[ericharper]

/ok to test b98698d

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