[docs] Clarify dtype defaults between trf v5 and TRL

[casinca]

What does this PR do?

This should Fix #5329
Feel free to tweak to your liking

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Note

Low Risk
Low risk documentation-only change that clarifies dtype inference/default behavior; no runtime code paths are modified.

Overview
Adds a note to docs/source/customization.md clarifying that Transformers v5 from_pretrained now infers dtype from the model config (rather than defaulting to float32), and advising users to pass dtype explicitly when needed.

Also documents that when TRL loads models from a model name string, it still defaults to float32.

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