[Relax][Frontend][TFLite] Add initial StableHLO builtin operator support

[Aharrypotter]

Summary

This PR adds initial Relax TFLite frontend support for 29 StableHLO builtin
operators from #19519 item I.

The covered subset includes pure elementwise ops, BuiltinOptions2 /
metadata-based ops, simple shape-manipulation ops, and a take-equivalent subset
of STABLEHLO_GATHER.

StableHLO builtins carry no TFLite-specific quantization or fused-activation
metadata, so the implementation uses dedicated converter helpers that bypass the
existing TFLite elemwise/QNN code paths.

Relates to #19519.

Changes

1. Zero-attribute elementwise helpers

   • Add _convert_stablehlo_unary, _convert_stablehlo_binary, and
     _convert_stablehlo_ternary for pure elementwise mapping.
   • Register 20 ops: unary (ABS, NEGATE, COSINE, EXPONENTIAL, FLOOR,
     LOG, LOGISTIC, RSQRT, TANH), binary (ADD, SUBTRACT, MULTIPLY,
     DIVIDE, MAXIMUM, MINIMUM, POWER), ternary (SELECT → R.where),
     and dtype-dispatched bitwise/logical ops (AND / OR → logical ops for
     bool or bitwise ops for integer, SHIFT_LEFT → R.left_shift for integer).

2. BuiltinOptions2 infrastructure

   • Add _get_stablehlo_options helper for parsing BuiltinOptions2 flatbuffers
     with enum validation via getattr(BuiltinOptions2, options_cls.__name__).
   • Register 6 ops: CONVERT → R.astype, CLAMP →
     R.minimum(R.maximum(...)), CONCATENATE → R.concat,
     BROADCAST_IN_DIM → R.reshape + R.broadcast_to, IOTA →
     R.arange + R.broadcast_to, and COMPARE → 6 comparison directions
     (TOTALORDER raises OpNotImplemented).

3. Shape-manipulation ops

   • PAD → R.nn.pad in constant mode. The initial PAD path supports
     non-negative edge padding with zero interior padding and a constant scalar
     padding value. Interior padding, negative padding, and dynamic padding
     values raise OpNotImplemented.
   • DYNAMIC_SLICE → R.dynamic_strided_slice. The initial path supports
     constant, in-bound start indices only. Runtime start indices and
     out-of-bounds StableHLO clamping semantics are deferred.

4. Indexing op

   • GATHER → R.take for the take-equivalent subset only.
   • Parses the relevant StablehloGatherOptions attributes needed to validate
     this subset: offset_dims, collapsed_slice_dims, start_index_map,
     index_vector_dim, and slice_sizes.
   • Validates the gather axis, collapsed dims, offset dims, slice sizes, and
     output shape against the expected R.take layout. Multi-dimensional and
     non-take-equivalent gather patterns raise OpNotImplemented.

5. Not included

   • STABLEHLO_RESHAPE, STABLEHLO_TRANSPOSE, and STABLEHLO_SLICE are left
     to another contributor who expressed interest in those ops.
   • The remaining Issue [Tracking Issue][TFLite] Remaining builtin operator coverage beyond #19412 #19519 StableHLO items are deferred to follow-up PRs:
     CBRT, REMAINDER, SCATTER, CONVOLUTION, DOT_GENERAL, REDUCE,
     REDUCE_WINDOW, DYNAMIC_UPDATE_SLICE, COMPOSITE, CUSTOM_CALL,
     RNG_BIT_GENERATOR, SORT, and WHILE.
   • More general or multi-dimensional STABLEHLO_GATHER patterns are also
     deferred to follow-up work.

Testing

All tests use manually-built minimal TFLite flatbuffers with
tvm.ir.assert_structural_equal. BuiltinOptions2 ops construct their options
via the FlatBuffers schema API, modeled after the existing DILATE test pattern.

python -m pytest tests/python/relax/test_frontend_tflite.py -k stablehlo -q

Result

• 29 StableHLO operators registered in the Relax TFLite frontend.

• 44 StableHLO test cases covering all registered ops, including
  structural-equal tests and unsupported/error-path checks:

  • COMPARE with TOTALORDER
  • PAD with interior padding, negative padding, and dynamic padding values
  • DYNAMIC_SLICE with runtime starts and out-of-bounds starts
  • non-take-equivalent or multi-dimensional GATHER

• All StableHLO TFLite frontend tests pass locally.

References

• Issue [Tracking Issue][TFLite] Remaining builtin operator coverage beyond #19412 #19519 item I: StableHLO operators in TFLite
• Related PR [Relax][Frontend][TFLite] Add DILATE operator mapping #19481: DILATE operator mapping, the first use of BuiltinOptions2
  in the TFLite frontend tests

[gemini-code-assist]

Code Review

This pull request implements support for a wide range of StableHLO operators in the TFLite frontend for TVM Relax, covering unary, binary, ternary, and more complex operations like gather and dynamic slice. The changes include the core conversion logic and comprehensive unit tests. Feedback points out a bug in a test helper function regarding FlatBuffers vector generation and suggests removing a redundant reshape call in the dynamic slice implementation.