Update detectron2/engine/defaults.py to make DefaultPredictor able to accept batch predictions

[irhawks]

Modify DefaultPredictor 's __call__ method to accept list of original_image at the same time keep this API compatible with previous code. (already checked with dev/linter.sh)

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[ywuenthought]

Does Detectron support real batch under the hood? Yes, you are feeding a batch of images to the predictor, but technically speaking, it still performs prediction one by one?

#282 (comment) illustrates the time cost between the one-by-one mode and the batch mode, and there is no significant difference.