rawPtr meaning and usage

Now, this function does two things, on CPU:

• Allocate a storage if it does not have one
• Returns a pointer, supposedly to this storage, which is meaningless except for CPU backend (#21 (closed)).

Though lazy initialization might be a good idea, it's not expected from the sole name of the function. Besides it cannot be generalized to other backend.

The main need for lazy initialisation is that actual tensor dimensions might not be known at tensor creation time but only when the full processing network, and possibly its inputs, is known. Yet lazy initialization might not be the better way to do this.