Tqdm bar has unexpected behavior when val_check_interval < 1
Describe the bug
If we set the trainer to have val_check_interval < 1, then here's what happens:

From batch_nb = 0 up to batch_nb = val_check_batch + 1: the tqdm bar correctly shows the progress on the training set.
Onwards: the validation step begins. Every batch gathered from the validation set is counted and added to the progress bar. So the progress bar will continue from val_check_batch + 1 up until val_check_batch + 1 + len(val_dataloader).

This is very counter intuitive and I suspect we can classify this as a bug! A more expected behavior would be to have a second tqdm bar spawning while the original training tqdm bar remains visible.
To Reproduce
Steps to reproduce the behavior:

Use show_progress_bar=True
Set val_check_batch to a value strictly below 1. The close to 0, the easier it will be to observe the bug.

Thanks! If I have a bit of time over the weekend, I'd be happy to try and make a PR.