ModelCheckpoint filenames and metric names containing / (slash)
🐛 Bug
The current behaviour of ModelCheckpoint is problematic when the checkpoint's name includes a metric whose name contains a slash character.
Since the implementation always includes the name of the metric along with its value, a format specifier like {some/name} results in the checkpoint being stored in a subdirectory. Everything appears to work correctly apart from the inconvenience of having a subdirectory which is not deleted when deleting old checkpoints.
Reproducer
https://colab.research.google.com/drive/1E4EKc2Ndt4XluRjpGC83hyohyF0BQESU?usp=sharing
The two lines marked as # STEP 1: ... and # STEP 2: ... are sufficient to reproduce the issue, the checkpoints are stored in lightning_logs/version_0/checkpoints.
Expected behavior
I haven't found a way to avoid this problem, because the substitution from {metric} to metric=value is hardcoded in PyTorch Lightning code.
I don't particularly care about the specific name of my checkpoints, so I'd propose for PL to automatically escape the slashes to some other character. Alternatively, PL should provide a way to manually specify a different name for the metric in the filename or a way to suppress the name altoghether.
Environment

CUDA:

GPU:

Tesla T4


available:         True
version:           10.1


Packages:

numpy:             1.18.5
pyTorch_debug:     False
pyTorch_version:   1.6.0+cu101
pytorch-lightning: 1.0.4
tqdm:              4.41.1


System:

OS:                Linux
architecture:

64bit



processor:         x86_64
python:            3.6.9
version:           #1 SMP Thu Jul 23 08:00:38 PDT 2020



Additional context
Supporting metrics with a slash in the name is desirable for me because wandb then automatically groups them in the dashboard.