Mhc optimization: Rewrites einsums for better xla fusion

[h1t35h]

Description

XLA optimizations does not seem to gel well with einsum for cases where dimensions are being expanded. Leading to a ~10- 15% performance drop in the current implementation. This change implements expanded einsums with jnp.expand_dims leading to better XLA fusion.

Before (110us):

After (97us):

Tests

All existing tests passing.

Checklist

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