Add eval_batch_size for evaluation

[NanoCode012]

Problem:

I would OOM during the evaluation stage despite setting micro_batch_size to be low. Turns out that batch size for eval using a different parameter.

Proposal:

New config for eval stage for lower VRAM GPUs.

Edit:

Since it's now a somewhat required argument, I'm not sure whether we should

• assert value exist
• set some default if not in config

[winglian]

thanks!