update pickling FAQ

[michaeldeistler]

See #1254

Sampling of old posteriors under v0.23.1 leads to errors. I don't think that these errors have an easy fix, so I am adding this to the FAQ.

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[8], line 1
----> 1 posterior.sample((100,), x=torch.ones((1, 3)))

File [~/anaconda3/envs/sbidebug/lib/python3.12/site-packages/sbi/inference/posteriors/direct_posterior.py:111](http://localhost:8888/lab/workspaces/~/anaconda3/envs/sbidebug/lib/python3.12/site-packages/sbi/inference/posteriors/direct_posterior.py#line=110), in DirectPosterior.sample(self, sample_shape, x, max_sampling_batch_size, sample_with, show_progress_bars)
    108 num_samples = torch.Size(sample_shape).numel()
    109 x = self._x_else_default_x(x)
    110 x = reshape_to_batch_event(
--> 111     x, event_shape=self.posterior_estimator.condition_shape
    112 )
    114 max_sampling_batch_size = (
    115     self.max_sampling_batch_size
    116     if max_sampling_batch_size is None
    117     else max_sampling_batch_size
    118 )
    120 if sample_with is not None:

File [~/anaconda3/envs/sbidebug/lib/python3.12/site-packages/torch/nn/modules/module.py:1729](http://localhost:8888/lab/workspaces/~/anaconda3/envs/sbidebug/lib/python3.12/site-packages/torch/nn/modules/module.py#line=1728), in Module.__getattr__(self, name)
   1727     if name in modules:
   1728         return modules[name]
-> 1729 raise AttributeError(f"'{type(self).__name__}' object has no attribute '{name}'")

AttributeError: 'Flow' object has no attribute 'condition_shape'

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.16%. Comparing base (6dec812) to head (d76c7d5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1255      +/-   ##
==========================================
- Coverage   86.05%   78.16%   -7.90%     
==========================================
  Files         118      118              
  Lines        8672     8679       +7     
==========================================
- Hits         7463     6784     -679     
- Misses       1209     1895     +686     

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