Need `dtype` attr implementation for xtype.NDArray

[ethancaballero]

dtype attr will go in xtype.NDArray dict in standard_method_map in myia.pipeline.resources; analogous to shape attr:
https://github.com/mila-iqia/myia/blob/master/myia/pipeline/resources.py#L210-L238

an example return of dtype would be Float[32] object