Fix regressor cost
Fix regressor cost

The regression loss function is wrong in tfvs. `tf.nn.l2_loss` returns a scalar (the sum of the per-example weights), which obviously will frequently get the weighted losses wrong. This PR calculates per-example losses, which are then weighted appropriately. (Sorry, this should have been fixed when tfvs was released.)
lilleswing/deepchem,Agent007/deepchem,peastman/deepchem,miaecle/deepchem,joegomes/deepchem,lilleswing/deepchem,Agent007/deepchem,joegomes/deepchem,miaecle/deepchem,lilleswing/deepchem,ktaneishi/deepchem,peastman/deepchem,deepchem/deepchem,deepchem/deepchem,rbharath/deepchem,miaecle/deepchem,ktaneishi/deepchem,Agent007/deepchem,rbharath/deepchem,ktaneishi/deepchem