Improve filename produced by Checkpoint when score value is Integer

[vfdev-5]

Following the comment: #745 (comment)

And BTW, I found some small bugs in current checkpoint handler:
The latest code added logic: suffix = "{}{:.4f}", which forces the "priority" to be float number.
There is some mixed use of " " and ' ', which causes the saved filename includes a ' '.
For example, the saved model filename for epoch 2 will be: 'checkpoint_epoch=2.0000.pth'

cc @Nic-Ma