Fix implementation of Transform.inv()

[allanzhao]

I've been porting some of my research code to Brax v2, and the performance is very good so far! Our code uses rigid body transforms extensively, and we noticed that Transform.inv() seems to differ from the textbook formula (as mentioned in #297).

This pull request:

1. changes the implementation of brax.v2.base.Transform.inv() to match tiny-differentiable-simulator and MuJoCo
2. adds a test case at the end of brax/v2/math_test.py (please feel free to move to an appropriate place)

[btaba]

Hi @allanzhao ! Thanks for the PR and documenting the bug!
We removed Transform.inv() in favor of inv_do

brax/brax/v2/base.py

Lines 127 to 129 in 9d5a88e

	def inv_do(self, o):
	"""Apply the inverse of the transform."""
	return _transform_inv_do(o, self)

the reason being that Transform.inv(), as implemented here, is wrt other Transforms, not necessarily wrt a Motion or a Force which is a bit confusing. We were only using Transform.inv_do(Motion), but if you want to change this into a Transform.inv_do(Transform) operator then that would be great!