[BUG] csv logger can't take "/" in metric names

Describe the bug

If the metric one is trying to log with the csv logger has / in its name, you will get a No such file or directory error.

To Reproduce

Run the sac example in examples/sac/sac.py. In config.yaml, set logger/backend to csv.

You will get an error at line 44 of torchrl/record/loggers/csv.py:

if filepath not in self.files:
    self.files[filepath] = open(filepath, "a")

Reason and Possible fixes

This is because the sac example logs train reward as "train/reward" where "train/" is taken by the logger as a path.

The csv logger can perhaps automatically set all / in metric names as "_" instead.