[SobolEngine] Fix edge case of dtype of first sample (#51578)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51578
https://github.com/pytorch/pytorch/pull/49710 introduced an edge case in which
drawing a single sample resulted in ignoring the `dtype` arg to `draw`. This
fixes this and adds a unit test to cover this behavior.
Test Plan: Unit tests
Reviewed By: danielrjiang
Differential Revision: D26204393
fbshipit-source-id: 441a44dc035002e7bbe6b662bf6d1af0e2cd88f4
