Support checkpointing for Sub-Epoch period 
Question
When setting period to a fractional value, checkpointing doesn’t trigger correctly. Additionally I think period should default to val_check_interval, if it doesn’t already.
To Reproduce
Steps to reproduce the behavior:
Run any model and set checkpoint to run at a fractional value. Only the first checkpoint will be saved.
Expected behavior
A checkpoint should be saved every specified period
Environment

Lighting Version: 0.9.0
PyTorch Version (e.g., 1.0): 1.6
OS (e.g., Linux): Ubuntu 16.04
How you installed PyTorch (conda, pip, source): pip
Build command you used (if compiling from source):
Python version: 3.7
CUDA/cuDNN version: 10.1