Add XTR retrieval and training support#207

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robro612 wants to merge 16 commits intolightonai:mainfrom

robro612:xtr-training

Commits on Mar 11, 2026

Add XTR retriever class that does XTR retrieval on supported indexes (non-plaid).
robro612
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Fix XTR retriever bugs, add imputation tests, and BEIR eval script
robro612
committed

Commits on Mar 18, 2026