What does this PR do?

Part of #13401

QwenEmbedRope.forward() copies pos_freqs and neg_freqs from CPU to GPU via .to(device) on every transformer forward call. These tensors are fixed at init and never change, so the repeated transfer triggers an unnecessary cudaStreamSynchronize (~76ms each).

Added _get_device_freqs() that caches the GPU copy on first call. Applied to both QwenEmbedRope and QwenEmbedLayer3DRope.

(register_buffer can't be used here because it drops the imaginary part of complex tensors)

Profiling (A100 80GB, eager, 2 steps, 1024x1024)

                                     BEFORE        AFTER
------------------------------ ------------ ------------
Big syncs (>50ms)                         3            0
Big sync total (ms)                   228.7          0.0
Big syncs before: [76.6, 76.4, 75.7]
Big syncs after:  []

Before (76ms cudaStreamSynchronize inside transformer_forward):

After (no sync gap):

Profiled with the tooling from #13356. Reproduction notebook.

Part of #13401

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Who can review?

@sayakpaul @dg845