Use automatic estimator batching by default

[RBendias]

Change the default estimator_batch_size for ICLModel.forward, ICLModel.fit, and the internal member execution path from 1 to "auto".

Explicit 1 remains available for minimum-memory sequential execution, and None still batches as many compatible estimators as possible. The automatic planner itself is introduced and tested separately in #1007; the benchmark integration is in #1008.

Depends on #1007 and #1008. Related: #989 and #994.

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