BatchSizeFinder safety margin

[edmcman]

Description & Motivation

My model allocates a bit more space for some examples than other. When I run BatchSizeFinder, the batch size it discovers works for a while, but eventually runs into a larger example and I run out of memory.

Pitch

I would like to add an argument to BatchSizeFinder that decreases the final batch size by a multiplicative factor. Possibilities are:

1. safety_margin of 0.1 would mean multiplying final batch size * (1-0.1)
2. scale of 0.9 would mean multiplying final batch size by 0.9

This would be turned off by default.

Alternatives

I implemented a pretty hacky version of this using inheritance:

    # Adds a safety margin.  For example, `safety_margin`  of 0.1 indicates that
    # the final batch_size will be reduced by 10%
    class SafeBatchSizeFinder(BatchSizeFinder):
        def __init__(self, safety_margin=0.1, *args, **kwargs):
            super().__init__(*args, **kwargs)
            assert safety_margin >= 0 and safety_margin <= 1.0
            self.safety_margin = safety_margin

        def scale_batch_size(self, trainer, *args, **kwargs):
            super().scale_batch_size(trainer, *args, **kwargs)
            original_batch_size = self.optimal_batch_size
            new_batch_size = int(self.optimal_batch_size * (1.0 - self.safety_margin))
            print(
                f"Found optimal batch size of {original_batch_size}, but with a safety margin of {self.safety_margin}, reducing it to {new_batch_size}"
            )
            self.optimal_batch_size = new_batch_size
            # This adjusts the data module batch_size.
            pl.tuner.batch_size_scaling._adjust_batch_size(trainer, value=new_batch_size)
            pl.tuner.batch_size_scaling._reset_dataloaders(trainer)
            trainer._active_loop.reset()

Additional context

I am willing to implement this. I don't think it would be hard.

cc @lantiga @Borda