Add helper function for detecting max. batch size

[SimonLarsen]

Should start from (optionally user-provided) guess and double/half until convergence.

Could be a CLI action e.g.

accelerate launch frogbox.find_batch_size \
    -c config.yml \
    --guess 32   # start from BS 32
    --divisor 8  # BS should be divisible with 4