Print Variable after tape has been backwarded

[kexinzhao]

There is a common user scenario that user may want to print the value of a grad var after running backward pass.

tape.Backward(loss)
LOG(INFO) << input->Grad().value();

However, since Value() will invoke tape.Forward() and Foward() will throw when the tape has been forwarded, so the above code will throw.

[tonyyang-svail]

@kexinzhao Thanks, this is a good catch.