Large `nact` number of autocorrelation lengths causes division-by-zero error with dynesty `act-walk` sampling method

When running PE on a BBH injection with dynesty, using the new act-walk sampling method, if the number of autocorrelation lengths nact is large, probably > 10, (in the example below, I used nact = 30), eventually a division-by-zero error gets thrown. This occurs because we thin the MCMC chain used to propose the next dynesty step by nact, but these chains will closer to ~a few autocorrelation lengths with the improvements to the computation of the autocorrelation length with the act-walk method. (Thanks for the help in understanding this issue @colm.talbot, please feel free to edit my remarks here.)

Code version: 15cb27e0 on dynesty-differential branch

Log snippet with dynesty settings:

22:48 bilby INFO    : Using sampler Dynesty with kwargs {'nlive': 100, 'bound': 'bilby', 'sample': 'act-walk', 'periodic': None, 'reflective': None, 'update_interval': 600, 'first_update': None, 'npdim': None, 'rstate': None, 'queue_size': 1, 'pool': None, 'use_pool': None, 'live_points': None, 'logl_args': None, 'logl_kwargs': None, 'ptform_args': None, 'ptform_kwargs': None, 'gradient': None, 'grad_args': None, 'grad_kwargs': None, 'compute_jac': False, 'enlarge': None, 'bootstrap': None, 'walks': 100, 'facc': 0.2, 'slices': None, 'fmove': 0.9, 'max_move': 100, 'update_func': None, 'ncdim': None, 'blob': False, 'save_history': False, 'history_filename': None, 'maxiter': None, 'maxcall': None, 'dlogz': 0.1, 'logl_max': inf, 'n_effective': None, 'add_live': True, 'print_progress': True, 'print_func': <bound method Dynesty._print_func of <bilby.core.sampler.dynesty.Dynesty object at 0x14b229430>>, 'save_bounds': False, 'checkpoint_file': None, 'checkpoint_every': 60, 'resume': False}
22:48 bilby INFO    : Checkpoint every check_point_delta_t = 600s
22:48 bilby INFO    : Using dynesty version 2.1.0
22:48 bilby INFO    : Using the bilby-implemented rwalk sampling tracking the autocorrelation function and thinning by 30 with maximum length 150000

Error trace:

Traceback (most recent call last):
  File "/Users/newolfe/Documents/projects/astro/ligo/ssm/sub-solar-mass-measurability/scripts/ssm-relbin-diffdynesty.py", line 87, in <module>
    result = bilby.run_sampler(
  File "/Users/newolfe/opt/anaconda3/envs/my_igwn-testing-local/lib/python3.9/site-packages/bilby/core/sampler/__init__.py", line 234, in run_sampler
    result = sampler.run_sampler()
  File "/Users/newolfe/opt/anaconda3/envs/my_igwn-testing-local/lib/python3.9/site-packages/bilby/core/sampler/base_sampler.py", line 96, in wrapped
    output = method(self, *args, **kwargs)
  File "/Users/newolfe/opt/anaconda3/envs/my_igwn-testing-local/lib/python3.9/site-packages/bilby/core/sampler/dynesty.py", line 483, in run_sampler
    out = self._run_external_sampler_with_checkpointing()
  File "/Users/newolfe/opt/anaconda3/envs/my_igwn-testing-local/lib/python3.9/site-packages/bilby/core/sampler/dynesty.py", line 578, in _run_external_sampler_with_checkpointing
    self.sampler.run_nested(**sampler_kwargs)
  File "/Users/newolfe/opt/anaconda3/envs/my_igwn-testing-local/lib/python3.9/site-packages/dynesty/sampler.py", line 1016, in run_nested
    for it, results in enumerate(
  File "/Users/newolfe/opt/anaconda3/envs/my_igwn-testing-local/lib/python3.9/site-packages/dynesty/sampler.py", line 838, in sample
    u, v, logl, nc = self._new_point(loglstar_new)
  File "/Users/newolfe/opt/anaconda3/envs/my_igwn-testing-local/lib/python3.9/site-packages/dynesty/sampler.py", line 395, in _new_point
    u, v, logl, nc, blob = self._get_point_value(loglstar)
  File "/Users/newolfe/opt/anaconda3/envs/my_igwn-testing-local/lib/python3.9/site-packages/dynesty/sampler.py", line 379, in _get_point_value
    self._fill_queue(loglstar)
  File "/Users/newolfe/opt/anaconda3/envs/my_igwn-testing-local/lib/python3.9/site-packages/dynesty/sampler.py", line 372, in _fill_queue
    self.queue = list(mapper(evolve_point, args))
  File "/Users/newolfe/opt/anaconda3/envs/my_igwn-testing-local/lib/python3.9/site-packages/bilby/core/sampler/dynesty_utils.py", line 178, in __call__
    self.build_cache()
  File "/Users/newolfe/opt/anaconda3/envs/my_igwn-testing-local/lib/python3.9/site-packages/bilby/core/sampler/dynesty_utils.py", line 310, in build_cache
    accept = max(accept // n_found, 1)
ZeroDivisionError: integer division or modulo by zero