Fix exception when sampling with post-selection

[cesarBLG]

Calling sinter.collect() for tasks including a postselection_mask raises an exception. This is caused because the decoder predictions are compared to the total number of shots without excluding discarded shots.

[Strilanc]

Could you add a unit test verifying this? I'm surprised it didn't trip existing unit tests to be honest; definitely something that should be tested.

[Strilanc]

Add a unit test with a postselected task that used to fail but now passes.

Something like taking 1 shot from

X_ERROR(1) 0
M 0
DETECTOR rec[-1]

[cesarBLG]

I added a unit test as suggested.

The reason it didn't trip any test is that circuits in _decoding_test.py are sampled using _sample_decode_helper_using_memory. The post-selection code in that function is not shared with the one in _CompiledStimThenDecodeSampler, so essentially the latter code isn't covered by any test.