Add rewrite for `log(gamma) -> gammaln`

[ricardoV94]

Description

We're missing this simple rewrite:

import pytensor.tensor as pt
from pytensor.graph import rewrite_graph

x = pt.scalar("x")
out = pt.log(pt.gamma(x))
new_out = rewrite_graph(out, include=("canonicalize", "stabilize", "specialize"))
new_out.dprint()

Can be done easily with PatternNodeRewriter as in

pytensor/pytensor/tensor/rewriting/math.py

Lines 3646 to 3651 in 911c6a3

	local_polygamma_to_tri_gamma = PatternNodeRewriter(
	(polygamma, 1, "x"),
	(tri_gamma, "x"),
	allow_multiple_clients=True,
	name="local_polygamma_to_tri_gamma",
	)

We could also add rewrites for common combinatorics expressions like

naive_betaln = pt.log((pt.gamma(x) * pt.gamma(y)) / pt.gamma(x + y)
betaln = pt.gammaln(x) + pt.gammaln(y) - pt.gammaln(x + y)

pytensor/pytensor/tensor/special.py

Lines 799 to 804 in ad55b69

	def betaln(a, b):
	"""
	Log beta function.
	"""
	return gammaln(a) + gammaln(b) - gammaln(a + b)

Or for log(poch):

pytensor/pytensor/tensor/special.py

Lines 767 to 772 in ad55b69

	def poch(z, m):
	"""
	Pochhammer symbol (rising factorial) function.
	"""
	return gamma(z + m) / gamma(z)

For these more general cases we can probably use something more flexible than the PatternNodeRewriter. We want to apply as long as we know all the terms inside are factorials/gammas/exps (positive things that easily blow up). This is a narrow/easier subset of #177