[model] fix: stuck issue with mixed text-image data

[HollowMan6]

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On top of #3496

Possibly resolve #3665

This PR continue addressing the hanging issue as mentioned here: #3315 (comment)

It should be pretty safe and optional to do model.training checks even the model is in inference mode, as the value of inputs_embeds won't get modified anyway.

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[gemini-code-assist]

Code Review

This pull request aims to fix a hanging issue with mixed text-image data by ensuring the vision part of the model is always exercised, even for text-only batches. This is achieved by removing the model.training condition, which triggers a dummy vision forward pass during inference as well. While this is a valid strategy to prevent deadlocks in distributed environments, it introduces a performance overhead for non-distributed inference. My review suggests refining this condition to apply the workaround only when in a distributed context, thus preserving performance for single-device inference scenarios.

[HollowMan6]

cc: @hiyouga @wlhgtc