Reduce tf.function retracing

[ymd-h]

This PR ensures input arguments of graph function (decorated by tf.function) as tf.Tensor instead of pure Python object, in order to reduce tf.function retracing.

Related with #99

[ymd-h]

@keiohta

If it is possible, please evaluate with more realistic tasks as well as simple tests.

We want to know whether there are good effects (speed up etc.) at TF2.3 and there are no bad effect (slow down, wrong answer etc.) at TF2.2 and before.

[keiohta]

Thanks for the PR! I'll merge it after checking whether the adding tf.constant deteriorate the performance in terms of time for computing forward and backward propagation of a network.

Unfortunately Anaconda has not supported gpu version of tf2.3 (which I typically use), so please wait for a moment.