Add trainer attribute to datamodule

[ananthsub]

What does this PR do?

Follow up to #3684. This adds an attribute on the datamodule for the trainer, which is later set in the training loop. It matches what's done for the lightning module: https://github.com/PyTorchLightning/pytorch-lightning/blob/master/pytorch_lightning/core/lightning.py#L92-L93

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[nateraw]

LGTM