Fix bit pack of mwpf and fusion blossom decoders under multiple logical observable

[yuewuo]

This PR fixed two bugs in MWPF decoder

1. Supporting decomposed detector error model

While MWPF expects a decoding hypergraph, the input detector error model from sinter is by default decomposed. The decomposed DEM may contain the same detector or logical observable multiple times, which is not considered by the previous implementation.

The previous implementation assumes that each detector and logical observable only appears once, thus, I used

frames: List[int] = []
...
frames.append(t.val)

However, this no longer works if the same frame appears in multiple decomposed parts. In this case, the DEM actually means that "the hyperedge contributes to the logical observable iff count(frame) % 2 == 1". This is fixed by

frames: set[int] = set()
...
frames ^= { t.val }

2. Supporting multiple logical observables

Although a previous PR #864 has fixed the panic issue when multiple logical observables are encountered, the returned value is actually problematic and causes significantly higher logical error rate.

The previous implementation converts a int typed bitmask to a bitpacked value using np.packbits(prediction, bitorder="little"). However, this doesn't work for more than one logical observables.
For example, if I define an observable using OBSERVABLE_INCLUDE(2) ..., supposedly the bitpacked value should be [4] because $1<<2 = 4$. However, np.packbits(4, bitorder="little") = [1], which is incorrect.

The correct procedure is first generate the binary representation with self.num_obs bits using np.binary_repr(prediction, width=self.num_obs), in this case, '100', and then revert the order of the bits to ['0', '0', '1'], and then run the packbits which gives us the correct value [4].

The full code is below:

predictions[shot] = np.packbits(
    np.array(list(np.binary_repr(prediction, width=self.num_obs))[::-1],dtype=np.uint8),
    bitorder="little",
)

[Strilanc]

Sorry about the CI breakages. Background radiation from dependencies churning beneath our feet...