Add more bijection transforms

[junpenglao]

The transformation currently available is a bit limited compare to tfp.bijector or numpyro.distributions.transforms, especially the ones for multivariate RVs (Cholesky, Cholesky for correlation matrix, ordered transformation, stickbreaking etc)

[rlouf]

It would be helpful if you could open an issue for each missing transformation!

[rlouf]

Here are the missing transforms I identified looking at other PPLs. We should open individual issues for each:

• [ ] Simplex constraint
• [ ] Zero-sum constraint
• [ ] Ordering constraint (at.sort)
• [ ] Truncation
• [ ] =at.abs=
• [ ] Cholesky
• [ ] L1BallTransform (stick-breaking)
• [ ] PermuteTransform (something subtensor)
• [ ] PowerTransform (at.power(x, a))
• [ ] SigmoidTransform (at.expit)
• [ ] Expm1 (at.expm1)
• [ ] Log1p (at.log1p)
• [ ] Softplus (at.softplus)
• [ ] Reshape