by zero with zero.

deleted good entries.

This was done in order to greatly simplify a new optimization to serialize
multiple IncSubtensors of a common base.  This is a case that happens if you
compute more than one subtensor from a single variable and then take a gradient.
Serialization helps because then all the IncSubtensors can be done inplace.

SetSubtensor.

1) Fixed __eq__ to work when other is not of same type as self.
2) Optimized TensorConstantSignature to compare data.sum() before comparing all elements.

different ids that compare equal.