trunk/717d7a288d82e60e92d7936cf41aaf3ec4b23c2d: torch/_native: route the chunked linear_cross_entropy ops (#195332)

What happened

This adds the torch._native override for the two chunked ops together with its eligibility condition, while the implementation still delegates to eager. Keeping the routing in its own commit leaves the kernel commit about the kernel. linear_cross_entropy puts a whole fused loss behind one dispatcher symbol: an (N, F) x (F, C) logits matmul, a row-shifted softmax, the cross-entropy reduction, and three gradient matmu…

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