`mode` of Evaluator can't work with string input

[Nic-Ma]

Describe the bug
Issue reported by @holgerroth :
this is for the saliency map inference. I need to run the evaluation in "training mode". SupervisedEvaluator in monai has that option but I guess it doesn't support the str type. it's checking for ForwardMode.EVAL or ForwardMode.TRAIN.

"/opt/monai/monai/engines/evaluator.py", line 208, in __init__
    super().__init__(
  File "/opt/monai/monai/engines/evaluator.py", line 123, in __init__
    raise ValueError(f"unsupported mode: {mode}, should be 'eval' or 'train'.")
ValueError: unsupported mode: train, should be 'eval' or 'train'.

FYI, without using train mode, I will get this error when using SaliencyInferer.