title: Implement for transforming rotation matrix to rotation vector in pytorch
body: <p>I want to implement the function for rotation matrix to rotation vector in pytorch. I have read the document in opencv tutorial <a href="https://docs.opencv.org/master/d9/d0c/group__calib3d.html#ga61585db663d9da06b68e70cfbf6a1eac" rel="nofollow noreferrer">Rodrigues</a> and refer the code in c <a href="http://read.pudn.com/downloads750/sourcecode/graph/opencv/2988718/Rodrigues/RodriguesTest.cpp__.htm" rel="nofollow noreferrer">Rodrigues in c</a>. The transformation for rotation vector to rotation matrix is straightforward according to the formulation, but the reverse is obscure and undirect.
  Is there any implement for the code in pytorch or any guide for implement this by myself?</p>