Confusing setting of validation frequency
🚀 Feature

Currently the validation frequency is set by check_val_every_n_epoch and val_check_interval of Trainer. The former one and latter one have an overlap in terms of frequency control actually. I propose use check_val_every_n_epoch and check_val_every_n_batches as the parameter:
check_val_every_n_epoch=10 is equivalent to check_val_every_n_epoch=10 previously
check_val_every_n_epoch=0.1 is equivalent to val_check_interval=0.1 previously
check_val_every_n_batches=10 is equivalent to val_check_interval=10.
And furthermore, these two options should be mutually exclusive. If one sets check_val_every_n_batches then shouldn't set check_val_every_n_batches.
Motivation
I propose this because I get confused when I try to tweak the validation frequency and find two options for setting it...