Allow batch ratio to be tuned

[ndb-rkang]

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Motivation

Current logic automatically determines batch ratio based on available VRAM. For certain complex, scanned PDFs, higher batch ratio leads to NPU OOM. Unclear if this is an issue with CUDA-based acceleration

Modification

Added an env var "MINERU_OVERRIDE_BATCH_RATIO" to tune batch ratio manually. Alternatively we can also change this to a ratio to be multiplied by vram-based batch ratio.

BC-breaking (Optional)

No - no backwards compatibility issue (unless MINERU_OVERRIDE_BATCH_RATIO is already used for some other application)

Use cases (Optional)

Processing scanned pdfs with high resource consumption

Checklist

Before PR:

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• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects.
• CLA has been signed and all committers have signed the CLA in this PR.

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[jon-snowman]

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