Negate the deprecated offline flag in CometLogger - #21921

The compatibility shim for the deprecated offline kwarg assigns it straight onto online:

# handle old "offline" experiment flag
if "offline" in kwargs:
    log.warning("The parameter `offline is deprecated, please use `online` instead.")
    if online is None:
        online = kwargs.pop("offline")

offline and online are opposites, so the flag is inverted. online flows to self._online and then to comet_ml.start(online=self._online):

CometLogger(offline=True)   ->  comet_ml.start(online=True)     # expected False
CometLogger(offline=False)  ->  comet_ml.start(online=False)    # expected True

CometLogger(offline=True) therefore starts a fully online experiment and uploads the run, which is the opposite of what the caller asked for. Before online was introduced in #20275 the code was self.mode = "offline" if offline else "online", so the inversion is exactly what the shim is meant to preserve.

The two neighbouring shims in the same block map synonyms — project_name onto project, save_dir onto offline_directory — where a direct assignment is right. This one maps antonyms, which is easy to miss.

Only callers still passing the deprecated offline kwarg are affected; anyone on online is unchanged.

Added test_comet_logger_deprecated_offline_flag, which checks both directions. It fails on master and passes here; tests/tests_pytorch/loggers/test_comet.py goes 8 -> 9 passing.

One thing worth flagging for reviewers: there is a separate bug just below at the disabled=True block (#21617, being fixed in #21619 / #21634). Until that lands, this change moves offline=True from "silently uploads online" to "silently disabled" rather than to a working offline experiment. That is still strictly better — no unintended upload — but the two want to land together to give offline=True its intended behaviour.