Add merge to NamedDataMap API

[lucylq]

🚀 The feature, motivation and pitch

Merge named data maps. Used for LoRA feature, when we have weights inside NamedData of the PTE and in the external file

Alternatives

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Additional context

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RFC (Optional)

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[lucylq]

PR: #12088

Initially templated on std::array, @JacobSzwejbka suggested removing this. This reduced size by 1kb (initially 6kb). I think we can further reduce size by reducing the scope of the MergedDataMap by not templatizing and removing some api implementations that won't be used by core.

Interestingly, we're only seeing a size regression internally, not in OSS. I can repro this locally with size_test.