Qiskit / qiskit-fermions / 34125176929

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refactor(circuit)!: build UCC from an ffsim UCCSD operator (#324)

Mirrors the UCJ change: the gate reimplemented the amplitude conventions and
the parameter-vector packing alongside the part that is actually this package's
concern, namely turning the amplitudes into a mapper-agnostic fermionic
circuit. The first half duplicated ffsim, so this narrows the gate to the
second half and takes the ffsim operator directly.

Verified rather than assumed: the removed `num_parameters`/`from_parameters`
agreed with ffsim's `n_params`/`from_parameters` (14 parameters at norb=4,
nocc=2, with identical t1/t2), and a restricted operator applied through this
gate reproduces ffsim's own state vector exactly on coupled-cluster amplitudes.

Unlike UCJ, this drops capability ffsim cannot currently replace, which is why
it is a separate commit: `from_t_amplitudes`, the `spinless` variant and the
opt-in `antisymmetric` parameterization all go. Users needing those can build
an Evolution over their own cluster operator, which also hands them control
over the Trotter ordering of its terms. Note that ffsim ships no Qiskit gate
for UCCSD at all, so this gate remains the only route from one of its UCCSD
operators to a circuit -- under Jordan-Wigner or any other encoding.

What remains is the part with no ffsim equivalent: the cluster generator and
its conjugate-paired grouping, which keeps every factor of the product formula
Hermitian and hence unitary. A term-by-term split would not even preserve the
norm, so the grouping tests are kept verbatim; the two that previously reached
this machinery only through the spinless variant are ported to the restricted
one, which exercises the same paths on a spinful register.

One pre-existing convention difference is now documented rather than changed:
the cluster operator only ever sees the part of a same-spin t2 that is
symmetric under the simultaneous exchange t2[i,j,a,b] = t2[j,i,b,a], while
ffsim reads the raw tensor. Coupled-cluster ... (continued)

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