Fix NaN value in Spline.log_q2r_parallel(q)

[pianwan]

I made two changes to log_q2r_parallel(q).

1. Use criterion = w / torch.abs(w + 1e-20) with + 1e-20 to avoid nan value.

   def log_q2r_taylor_w(w, theta):
   criterion = w / torch.abs(w + 1e-20)
   return criterion * torch.pi / theta

2. Use torch.where() to replace values where theta < eps_theta and w < eps_w & theta >= eps_theta.

   def log_q2r_parallel(q, eps_theta=1e-20, eps_w=1e-10):
   x, y, z, w = q[..., 0], q[..., 1], q[..., 2], q[..., 3]
   
   theta = torch.sqrt(x ** 2 + y ** 2 + z ** 2)
   
   bool_criterion_theta = (theta < eps_theta)
   bool_criterion_w = ((theta >= eps_theta) & (torch.abs(w) < eps_w))
   
   taylor_theta = log_q2r_taylor_theta(w, theta)
   taylor_w = log_q2r_taylor_w(w, theta)
   normal = log_q2r(w, theta)
   lambda_ = torch.where(bool_criterion_theta, taylor_theta,
                         torch.where(bool_criterion_w, taylor_w, normal))
   
   r_ = torch.stack([lambda_ * x, lambda_ * y, lambda_ * z], -1)
   
   return r_

There will be seemingly no NaN value.