feat: add epoch_key to CometLogger to make epoch extraction configurable

[pupeno]

What does this PR do?

Contributes to #21782

Adds epoch_key to CometLogger to let users control which incoming metric key is extracted
and forwarded to Comet's dedicated epoch= argument.

Previously this key was hardcoded to "epoch". With this change it is configurable, and can be
set to None to disable epoch extraction entirely.

Default is "epoch", which preserves the previous behaviour — fully backward-compatible.

# before: always extracted "epoch" from metrics
logger = CometLogger(...)

# after: same default, no change
logger = CometLogger(...)

# extract a prefixed epoch key (e.g. when used with Trainer(log_key_prefix="trainer/"))
logger = CometLogger(epoch_key="trainer/epoch")

# disable epoch extraction entirely
logger = CometLogger(epoch_key=None)

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