Qiskit / qiskit / 35887581685

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Add Dim::Bound variant and reorganize the tensor module (#16914)

* Bound the dynamic axes of a tensor type

A shape axis is either a fixed size or a size bounded above, in the manner of
StableHLO's bounded dynamism, so storage can be sized from the bound while
the size actually used stays a run-time fact. This replaces an axis named for
a later pass to resolve.

Type-level rules move to tensor::rules, where promotion, require_static and
broadcast_dims answer what an operation would do to a pair of types with no
values in hand. broadcast_dims requires a true size per axis pair rather than
over whole shapes, so a bounded axis forwards through a size of 1 and is
rejected only where it meets a size it would have to be compared against. It
shares the right-aligned axis pairing with broadcast_shape, so the two cannot
disagree about which axes meet.

A Dim renders as 4000 or <=2 and the two new TensorError variants render
shapes through it, so an error over declared shapes reads properly. A Tensor
compares for equality directly, and TensorType::concrete_shape routes through
require_static so that one walk over a shape answers both questions.

* Give the tensor type domain a file per concept

A mod.rs is a door, not a definition site, and tensor/mod.rs had grown to
1,100 lines of definitions. Move the type side out of it: dtype.rs for the
element type, tensor_type.rs for a shape of per-axis sizes and the type that
pairs one with a dtype, and error.rs for the domain's error set.

error.rs rather than folding TensorError into the tensor value: of its four
variants the value side constructs only DTypeMismatch, and the other three
belong to broadcasting and to the type-level rules, so no single concept owns
the enum. fmt_shape becomes pub(super) for it, and three doc links become
qualified now that Tensor and rules are no longer in the same scope.

Pure movement: the tests move with the items they cover and the test count is
unchanged.

* Reduce the tensor modul... (continued)

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