allow val_check_interval to be larger than training dataset size 
🚀 Feature

allow val_check_interval  to be larger than the number of the training batches in one epoch
Motivation
I am using a small datasets, so instead of specifying max_epochs in Trainer, I want to use max_steps and evaluate every val_check_interval , but when val_check_interval is larger than number of batches in training set, there is an error, like this:
`val_check_interval` (100) must be less than or equal to the number of the training batches (24). If you want to disable validation set `limit_val_batches` to 0.0 instead.

Pitch
val_check_interval  should't be limited by the number of the training batches in one epoch, it should be a warning, not an error
Alternatives
I am currently using a wrapper to make it an iterable dataset, so it allows me to do that
Additional context