ENH: Nested rhat MCMC diagnostic

[gil2rok]

Implement nested R-hat for Markov chain Monte Carlo (MCMC) diagnostic.

The potential scale reduction factor, also known as R-hat, is a popular MCMC diagnostic from Gelman and Rubin.
R-hat detects convergence of MCMC chains by comparing within chain variance to between chain variance.

Nested r-hat from Margossian et al. better predicts convergence when running thousands of short chains on modern hardware. Nested r-hat uses superchains, collections of MCMC chains, and compares within and between chain and superchain variance.

I am seeking feedback on the code style + API design. The code is somewhat complicated by requiring input_array to have 4 dimensions -- num_superchains, num_chains, num_samples, and num_params -- where most users may expect only 3 (or 2). Tests are also still needed, as well as a brief doc explanation of the math.

Quick nit: why does the existing R-hat function return the potential scale factor after flattening along the sample and chain dimensions? I followed this convention for my implementation.

Addresses issue #278 .

[gil2rok]

@charlesm93: you may be interested in this.

(For everyone else: Charles is the author of the nested R-hat paper.)

[gil2rok]

Code style tests are failing because Flake8 is finding extra spaces around operators on line 43 of smc/resampling:

However I do not see what the problem is. Here's the line:

blackjax/blackjax/smc/resampling.py

Line 43 in 65ae00e

"""

If anyone sees how to fix this issue, please let me know.

[junpenglao]

Thanks! Great start. Let me know when you add some test.

[junpenglao]

You should relax the ndim, as our input could have multiple dimensions of event shape (ie the random variable is non-scaler).

[junpenglao]

Use keepdims=True and it should works.

[junpenglao]

Quick nit: why does the existing R-hat function return the potential scale factor after flattening along the sample and chain dimensions? I followed this convention for my implementation.

The current r-hat function does not flatten the sample, but rather squeeze it, so if you have a random variable with shape=(2, 5), the output result could be
shape=(1, 1, 2, 5) or (1, 2, 5, 1)
doing a squzze makes it return rhat the same shape as the random variable.

[gil2rok]

Thanks so much for the quick feedback! I'll continue working on this next week.