Fix interval merge invariant in _find_disjoint (#47860)
Fix interval merge invariant in `_find_disjoint`
`_find_disjoint` implements a sweep-line merge over tensor memory ranges,
but assigns `last_stop = stop` instead of tracking the running maximum.
When a tensor is strictly contained inside an earlier, larger one,
`last_stop` shrinks to the contained tensor's end pointer, so a subsequent
tensor that genuinely overlaps the larger one is classified as disjoint.
Use `last_stop = max(last_stop, stop)`, the standard sweep-line invariant.
Note on impact: this is a latent correctness fix, not a user-facing bug fix.
Triggering the mis-classification requires strict containment, which in turn
guarantees the resulting group is not "identical" in `_find_identical`, so
`remove_tied_weights_from_state_dict` raises the same RuntimeError either way.
Verified exhaustively over all 3- and 4-interval configurations (54k cases):
the corrected version matches ground-truth connected components in 100% of
cases, and the raise-vs-succeed outcome never differs. The observable
improvement is that the error message now reports the complete set of
overlapping tensors instead of a truncated one.
