[CPU/WebGPU] Support empty MatMulNBits weights (#33559)
### Description
Support empty packed weights in CPU and WebGPU `MatMulNBits`. With
`weight_prepacked=0`, one or more dimensions of B's packed shape may be
zero; the other dimensions must retain their normal values.
The output keeps the logical shape determined by A, K and N, and
contains zeros or the broadcast bias. Unused quantization parameters may
also be empty. CPU skips unused parameter packing and matrix
multiplication; WebGPU records a fill dispatch so graph replay rewrites
the output correctly. QKV/MLP fusion remains enabled: when a projection
has empty weights, the fused operator uses the shared zero/bias fallback
while computing the nonempty projections.
Includes operator documentation and focused tests for FP16/FP32,
2/4/8-bit weights, constant/runtime inputs, bias, malformed shapes,
empty/nonempty overrides, and graph capture.
### Motivation and Context
Optional projections, such as disabled LoRA adapters, should not require
full-size zero weight tensors. Zero-sized defaults let the same raw
`MatMulNBits` graph run on CPU and WebGPU, with nonempty weights
supplied when needed.
Split from #33370. That PR retains these changes until this PR merges;
QLoRA-specific operators, fusion and shaders remain there.
### Validation
Built the standalone branch against main with built-in WebGPU on Windows
x64. All 16 empty-weight tests, 28 existing QKV/MLP fusion regressions,
and 8 existing nonempty-weight regressions passed; 4 Arm64-specific
tests were skipped. Coverage includes fused empty/nonempty overrides,
MLP bias, residual outputs, and capture replay. FP16 WebGPU cases
require confirmed ShaderF16 support. Lint and `git diff --check` passed.
