fix DBFS file path problem on Dataricks

[PatrickkZ]

Description

when call

orca_estimator = Estimator.from_torch(model=model_creator,
                                      optimizer=optimizer_creator,
                                      loss=criterion,
                                      metrics=[Accuracy()],
                                      model_dir=model_dir',
                                      use_tqdm=True,
                                      backend=backend)

on Databricks cluster, model_dir should be a shared path to save files, which is a path starts with /dbfs or dbfs:/ on Databricks. but currently, the DBFS format path is not support yet, this PR fix this problem. This is verified on Databricks.