slim down progress bar default printing
🚀 Feature
The progress bar is cluttered, too many prints by default. Let's remove some of them.
Pitch
Hi guys,
I feel the progress bar contains some uninformative prints and leaves little space for user customization.
This is the progress bar print before validation, with empty tqdm_dict (so no user addition to the progress bar metrics):
Epoch 1:  39%|███▉      | 343/875 [00:07<00:10, 48.46batch/s, batch_nb=342, gpu=0, loss=0.408, v_nb=18]  

This is after a validation step where I add to the tqdm_dict validation accuracy and loss:
Epoch 2:  20%|█████████████████▏                                                                      | 171/875 [00:03<00:11, 58.99batch/s, batch_nb=170, gpu=0, loss=0.267, v_nb=21, val_acc=91.8, val_loss=0.278]

To avoid more clutter and make some room for user-defined metrics, I propose to remove the following outputs from the progress bar:

batch_nb: this is redundant, essentially the same number is printed on the left
loss: this is something always computed in the training step. If the user wants to show it, he just adds it to the tqdm_dict
gpu: this info is useless in single-gpu setting, and likely also in multi-gpu ones.
v_nb:  This is the experiment version number, which is very convenient to know. Since it stays the same during the whole experiment, we could just print it by default before the training starts.

Also, I propose to rename "batch/s" to tqdm default "it/s"  to save a few extra characters.
Pinging @tullie @Borda since this discussion almost started in #531 .
Best,
C