Add ComplexTransform and update biject_to registry.

[tillahoffmann]

I added a complex constraint in #1762 with biject_to using the identity function as shown below. This makes inference challenging in practice because we end up with a mix of real and complex parameters.

numpyro/numpyro/distributions/transforms.py

Lines 1650 to 1654 in 93e11c2

	@biject_to.register(constraints.complex)
	def _transform_to_complex(constraint):
	return IdentityTransform()

This PR adds a ComplexTransform which turns a real tensor of shape (..., 2) into a complex tensor of shape (...). It also updates the biject_to registry such that complex variables can be integrated into models (e.g., for FFTs) while ensuring that the unconstrained space is real.

[fehiepsi]

LGTM, thank you!