fix quaternion's gradients in PoseInverse, and a few other warp kernels

[YuhengZhi]

1. The current conversions from wp.quaternion to wp.vec4 would stop gradients from properly propagating. This has caused the issue mentioned in #392.

   Instead of

   out_v = wp.vec4()
   out_v[0] = out_q[3]
   out_v[1] = out_q[0]
   out_v[2] = out_q[1]
   out_v[3] = out_q[2]

   , this commit does the conversion from quaterniom to vec4 as

   out_v = wp.vec4(out_q[3], out_q[0], out_q[1], out_q[2])

   which seems to produce correct results in the example used in #392 .

2. A few other wp.kernel's in the same file were using the same conversion method, so are all modified accordingly.

3. Fixed missing/extra None's in a few backward functions, mentioned in #382 .

Please feel free to let me know if there are any issues.

[balakumar-s]

Thanks for the fix!