[DistMuon] Drop the foreach-copy compatibility fallback - #4271

``_foreach_copy_or_fallback_`` checks four conditions before using
``torch._foreach_copy_`` and copies pair-by-pair when any fails. Three of the
four are unnecessary: the op casts between dtypes, crosses devices, and handles
non-contiguous views exactly as ``Tensor.copy_`` does. Only unequal shapes are
a real difference -- ``copy_`` broadcasts, the foreach op raises -- and callers
pair each destination with a source built from the same ``_TensorRegion``, so
their shapes always agree. When they do not, the op's own error is clearer than
silently taking a slower path.

Measured on Moonlight-16B-A3B, DP-shard 8 with EP 4 over 8xH100, 10 steps: the
fallback ran 0 times out of 3220 multi-pair calls, and no condition failed
once. Every batched call held exactly 8 uniform pairs.

Rename to ``_batched_copy_``: there is no longer a fallback to name, and
reusing ``_foreach_copy_`` would shadow the torch op this calls and the tests
patch.

Test plan:
- pytest tests/unit_tests/flex_shard/test_optimizer_reshard_runtime.py -- 6
  passed. The dtype test now asserts the foreach path is taken and still casts,
  which is the evidence for removing the guard; a new case covers the shape
  mismatch the op rejects.
- Bit-identical numerics against the parent commit on the config above with
  --debug.seed 42 --debug.deterministic: step 1 loss 12.47077 grad_norm 1.1954,
  step 10 loss 10.69065 grad_norm 1.9209.

ghstack-source-id: 3379b3e
Pull-Request: #4271