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Comparison between Keras and Estimators (tf.estimators):
          i) Estimators.
            i.a) Estimators are based on arbitrary code with arbitrary control flow and so it's quite tricky to force any structure onto them.
            i.b) Estimators support 3 modes - train, eval and predict. Each of these could in theory have completely independent flows, with different weights, architectures etc. This is almost unthinkable in Keras and would essentially amount to 3 separate models.
          ii) Keras.
            ii.a) Keras supports 2 modes - train and test, which is necessary for things like Dropout and Regularisation.

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