Implement log_graph for CometLogger. (#5295)

Co-authored-by: chaton <[email protected]>

pytorch_lightning/loggers/comet.py

@@ -25,6 +25,7 @@
 from torch import is_tensor
 
 from pytorch_lightning import _logger as log
+from pytorch_lightning.core.lightning import LightningModule
 from pytorch_lightning.loggers.base import LightningLoggerBase, rank_zero_experiment
 from pytorch_lightning.utilities import rank_zero_only, _module_available
 from pytorch_lightning.utilities.exceptions import MisconfigurationException
@@ -310,3 +311,7 @@ def __getstate__(self):
         # needed later
         state["_experiment"] = None
         return state
+
+    def log_graph(self, model: LightningModule, input_array=None) -> None:
+        if self._experiment is not None:
+            self._experiment.set_model_graph(model)