… per-shader timing

Pull Request resolved: #17105

This change improves the benchmark test harness in three ways:

1. **Reference computation caching**: Test cases are now grouped by a
   `ReferenceKey` that captures the inputs affecting reference output
   (sizes, dtype, data generation type). Reference computation runs
   once per group and results are reused, significantly speeding up
   test suites with many storage/layout variations of the same logical
   test case.

2. **Per-shader timing breakdown**: Benchmark output now shows
   individual shader execution times with global and local workgroup
   sizes, making it easier to identify performance bottlenecks when
   multiple shaders participate in an operator.

3. **Deferred data generation**: Tensor data is now generated lazily
   with explicit seeding, enabling deterministic data sharing across
   grouped test cases. This ensures identical inputs produce identical
   reference outputs for caching correctness.

Also adds string input support (`ValueSpec::make_string()`) and helper
functions for concise test case naming (`layout_abbrev`, `repr_str`,
`shape_string`).

ghstack-source-id: 338638546
@exported-using-ghexport

Differential Revision: [D91945038](https://our.internmc.facebook.com/intern/diff/D91945038/)

…perators (#17261)

Implemented quantize_per_tensor and dequantize_per_tensor GLSL shaders
and C++ dispatch logic to support the new single-dimension packed INT8 layouts
(kPackedInt8_4W, kPackedInt8_4C, kPackedInt8_4H). These operators enable
conversion between floating-point tensors and packed int8 representations with
per-tensor scale and zero-point parameters.

The implementation includes:
- GLSL shaders: quantize_per_tensor and dequantize_per_tensor with support for
  both texture->buffer and buffer->buffer data flows, including GL_EXT_debug_printf
  statements for debugging
- QuantizeDequantize.cpp: Added dispatch functions for the new layouts and
  registered etvk.q_dq_8bit_per_tensor.default operator
- Test infrastructure: Created q_dq_8bit_per_tensor test binary with DEBUG_MODE
  support and reference CPU implementation for validation

The shaders implement the quantization formula Q = clamp(round(x/scale) + zp, -128, 127)
and dequantization formula x' = (Q - zp) * scale, with proper int8 packing/unpacking
using little-endian byte ordering and sign extension.

Differential Revision: [D92061370](https://our.internmc.facebook.com/intern/diff/D92061370/)

[ghstack-poisoned]