fix(gpt-neox): preserve rotary_pct across save/load cycle

[s-zx]

Summary

GPTNeoXConfig.convert_rope_params_to_dict unconditionally overwrote
rope_parameters["partial_rotary_factor"] with the default 0.25 when
rotary_pct was absent from kwargs.

On every from_pretrained call, rotary_pct is not present as a top-level
key in config.json (it is stored inside rope_parameters), so the value was
silently reset to 0.25.

Fix

Replace the unconditional assignment with a conditional + setdefault:

-self.rope_parameters["partial_rotary_factor"] = kwargs.pop("rotary_pct", 0.25)
+rotary_pct = kwargs.pop("rotary_pct", None)
+if rotary_pct is not None:
+    self.rope_parameters["partial_rotary_factor"] = rotary_pct
+else:
+    self.rope_parameters.setdefault("partial_rotary_factor", 0.25)

When rotary_pct is absent (the reload path), setdefault only fills in 0.25
if partial_rotary_factor is not already present, so an existing value is preserved.

Fixes #44913

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: gpt_neox

[Rocketknight1]

Hi @s-zx, no more code agent PRs like this please! As we state in CONTRIBUTING, the reviewers are getting flooded with these, and usually they have issues that require extra maintainer attention. For example, your other PR at #44916 broke the copying mechanism, which was being discussed in the issue at #44855 when you opened the PR.