Add block-spin update sampler

[kevinchern]

This PR introduces a block-spin update sampler.
It includes duplicate code from #40

[jackraymond]

Great job, I'll come back to this when you add tests. Per your summary. Thanks for getting this draft up quickly.

Would be nice to have a tidy public GPU code to use and cite for speedup relative to our CPU implementation in higher throughput applications as soon as possible.

[kevinchern]

Samplers should have a consistent interface. Related: #52, #47

[kevinchern]

@jackraymond I updated the PR:

1. added tests (some notable mentions below)
   • test gibbs update by checking magnetization is close to expected
   • test metropolis update by checking magnetization is close to expected
   • test metropolis update by checking oscillatory behavior for beta = 0 case
2. added documentation (first pass)
3. capitalized Metropolis/Gibbs and also make it case-insensitive
4. The sampler has a simplified signature. It no longer tries to do the colouring itself. Instead, it requests for a colouring function. This generalizes it beyond CPZ topologies. One could, for example, default to a colouring heuristic when no colouring is given, but I've opted away from that for modularity.
   5. Now a metadata dictionary is returned along with the sample.

[VolodyaCO]

I left some comments about form.

Overall looks fantastic. Thank you.

[VolodyaCO]

Can the user compile a method of an existing nn.Module subclassed instance? Ideally, we would want the user to decide whether they want to compile the _step method. It seems as though torch.compile(module) compiles the __call__ method, so maybe we can wrap _step in it and time the compiled and non-compiled versions.

[kevinchern]

Addressed @thisac and @VolodyaCO 's PRs. Briefly,

1. Remove inheritance of nn.Module
2. Add functional module docstring
3. Add explicit named args for, and renamed to randspin
4. Improved documentation and type-hinting

[jackraymond]

Looks good tests ran for me.

I'd recommend you allow setting of x as an initial condition. Check it doesn't move when num_sweeps is none, and check it oscillates under BlockMetropolis when temperature is infinity as tests perhaps.

[kevinchern]

Looks good tests ran for me.

I'd recommend you allow setting of x as an initial condition. Check it doesn't move when num_sweeps is none, and check it oscillates under BlockMetropolis when temperature is infinity as tests perhaps.

In the latest commit, I made the following changes:

• Fixed documentation RE nonzero & finite temperature regimes.
• Allow setting of initial states.
  • Test by checking sampler.x.tolist() is equal to input initial states list
• Added custom errors to distinguish from ValueError

@jackraymond @thisac

[VolodyaCO]

All comments have been addressed. Thank you @kevinchern

[thisac]

Thanks @kevinchern! 🎄

[thisac]

Better put in a setUpClass() method.

[kevinchern]

If I understood correctly, that would be incompatible with @parameterized.expand