GEOPY-2910: Reduce chunking of sensitivities for TEM inversions

[domfournier]

GEOPY-2910 - Reduce chunking of sensitivities for TEM inversions

[Copilot]

Pull request overview

This PR targets GEOPY-2910 by adjusting how TEM sensitivity computations are chunked/assembled in the Dask TDEM simulation path, aiming to reduce excessive chunking overhead during inversions.

Changes:

• Disables get_parallel_blocks(..., optimize=False) to avoid extra re-splitting of blocks.
• Refactors get_field_deriv_block to assemble a single per-block matrix of back-solved field derivatives rather than a list of per-chunk arrays.
• Updates compute_rows to slice the new per-block field-derivative matrix using a running column offset (colm_count).

Comments suppressed due to low confidence (1)

simpeg/dask/electromagnetics/time_domain/simulation.py:480

• When local_ind is empty you continue without advancing colm_count, so subsequent receiver chunks will slice the wrong columns out of field_derivs[block_ind]. colm_count needs to be incremented even when a chunk has no active (time-masked) entries.

        if len(local_ind) < 1:
            row_block = np.zeros(
                (len(ind_array[1]), simulation.model.size), dtype=np.float32
            )
            rows.append(row_block)
            continue

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