Backport `pm.logp` and related functional syntax to `v3`

[michaelosthege]

Description of your problem

To reduce the rate of users running into problems like #5048, we should backport the pm.logp, pm.logcdf etc syntax to v3.

While it will of course not have all the features available in v4 (for example doing pm.logp() on things other than what's returned by Distribution()), the availability of the new syntax allow us to replace all uses of rv.logp(x) with forwards-compatible pm.logp(rv, x) (like #5051).

The actual implementation may be as simple as:

def logp(rv, x):
    return rv.logp(x)

Versions and main components

• PyMC3 Version: v3.11.x

[canyon289]

For other examples of why this cohesiveness is desirable see related issue #5052 and #5048

[michaelosthege]

The backport of pm.logp was merged to v3 which does not automatically close issues.