Improve the default ProgressBar
🚀 Feature

It would be great to have different bars to individually indicate different training stage, such as

train_progress_bar: display the train_loop
val_progress_bar: display the val_loop
main_progress_bar: display max_epochs*(train_loop + val_loop)
test_progress_bar: display the test loop

Motivation

The current default ProgressBar has some flaws:

using train_progress_bar to display the train_loop+val_loop, which may get user confused, people are unaware of when the train_loop is finished
it can't display the ETA of total training process

Pitch

just like this pic demostrates:

Alternatives

Additional context