function BaseCallback() {
	    // TODO(michaelterry): This type is a best guess.
	    this.validationData = null;
	  }
Abstract base class used to build new callbacks.

The `logs` dictionary that callback methods take as argument will contain
keys for quantities relevant to the current batch or epoch.

Currently, the `.fit()` method of the `Sequential` model class
will include the following quantities in the `logs` that
it passes to its callbacks:

onEpochEnd: Logs include `acc` and `loss`, and optionally include `valLoss`
  (if validation is enabled in `fit`), and `valAcc` (if validation and
  accuracy monitoring are enabled).
onBatchBegin: Logs include `size`, the number of samples in the current
  batch.
onBatchEnd: Logs include `loss`, and optionally `acc` (if accuracy monitoring
  is enabled).