fix `scalar_to_array` return type

[ethancaballero]

"So we kind of need to figure something out here, because the reference Python implementation of scalar_to_array does not instantiate an array that matches the current frontend or backend.

It would probably be more robust to change scalar_to_array to take a type argument specifying which array class to use. The user would never call it directly, but functions like numpy.array would fill in the argument appropriately."

Originally posted by @breuleux in
https://github.com/mila-iqia/myia/pull/230/files/866930ec5882f9eaea7728985d8a1be4261d0b00#diff-6a05bcaf0b55a60e525920f65f3d51b2