Gradient Accumulation Scheduler
Is your feature request related to a problem? Please describe.
Often during a training, loss changes really rapidly first few epochs, so, at this time, we don't really need to use a gradient accumulation. And sometimes we want to schedule changing of accumulation factor, for example, increase it every 10 epochs
Describe the solution you'd like
Let's define scheduler, what will control the changing of accumulation factor
schedule = {6:2, 11:4} 
accumulator = GradientAccumulationScheduler(schedule)
According to this schedule, we will fit our model first 5 epochs with factor 1. next 5 with factor 2, and next epochs a factor will be 4
Describe alternatives you've considered

We can interrupt model finning and manually change factor, but its really not user-friendly
We can override on_epoch_begin in pl_model, and changing factor this way.

Additional context
I have heard about this technique at ML Training from one of the competition winners, so it could be a useful feature