Feature: Control number of vmapped envs in evaluator using `arch.num_envs`

[OmaymaMahjoub]

What?

Modify the evaluator to limit the number of vmapped envs to arch.num_envs when the total number of evaluation episodes arch.num_eval_episodes exceeds this limit (instead of parallelise all the arch.num_eval_episodes). In such cases, evaluations are conducted in sequential batches, with each batch containing arch.num_envs parallel envs.

Why?

Limiting parallel evaluations to num_envs prevents out-of-memory issues by avoiding vmap over all episodes at once.