[Relax][Frontend][TFLite] Support dynamic RANGE scalar bounds

[Aharrypotter]

Summary

This PR adds Relax TFLite frontend support for dynamic (runtime) scalar bounds
in the RANGE operator, addressing the RANGE "fix partial implementations"
item from #19412 section C.

convert_range previously lowered only constant start, limit, and
delta to relax.op.arange and raised OpNotImplemented for runtime scalar
bounds (the guard added in #19401). Models that compute RANGE bounds at runtime
could therefore not be imported. This PR makes the dynamic path work for both
integer and float bounds, ascending or descending, without adding a new Relax
op. The change is limited to the RANGE converter and its test.

#19813 added a batch of missing TFLite operator mappings but did not touch this
partial-implementation item; this PR closes it.

Design

Dynamic scalar bounds via count-lift

relax.op.arange only accepts compile-time PrimExpr bounds. The frontend
already has a runtime-scalar -> symbolic-dimension bridge
(relax.op.tensor_to_shape + match_cast, as used by
_get_shape_expr_from_tensor), so no new op is needed.

Rather than feed symbolic bounds straight into arange, the converter computes
the element count in-graph and lifts that single value to one symbolic
output dimension L, then rebuilds the values as arange(0, L) * delta + start.
Lifting the count (instead of the bounds) keeps the declared and runtime output
lengths equal by construction: arange's struct-info length formula
(InferTypeArange) has no negative-step branch, so feeding symbolic bounds
directly would mis-declare descending ranges relative to the TOPI runtime
length.

The count is max(0, ceil((limit - start) / delta)), computed per dtype:

• integer: -floor_divide(start - limit, delta) — exact, sign-agnostic, and
  free of float-precision loss; equal to ceil((limit - start) / delta).
• float: ceil((limit - start) / delta).

Constant (all-bounds-constant) RANGE keeps the existing direct-arange path
unchanged.

Operator Support

Operator	TFLite inputs	Relax lowering	Supported subset
RANGE	scalar start, limit, delta	relax.op.arange (constant bounds); count-lift + arange(0, L) * delta + start (dynamic bounds)	int and float, constant or runtime scalar bounds, ascending or descending

Tests

The dynamic test compiles the imported module and runs it on the Relax VM,
comparing the output against numpy.arange. The constant-bound structural test
is unchanged.

Test	Coverage
test_range	constant scalar bounds (existing, unchanged)
test_range_dynamic_scalar_inputs	runtime scalar bounds: int and float, ascending and descending

Local validation:

python -m ruff format --check \
  python/tvm/relax/frontend/tflite/tflite_frontend.py \
  tests/python/relax/test_frontend_tflite.py

python -m ruff check \
  python/tvm/relax/frontend/tflite/tflite_frontend.py \
  tests/python/relax/test_frontend_tflite.py

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

python -m pytest \
  tests/python/relax/test_frontend_tflite.py -q

Result:

ruff format --check: 2 files already formatted
ruff check: All checks passed
range tests: 12 passed, 536 deselected
full TFLite pytest: 548 passed

References

• Issue [Tracking Issue][TFlite] Expand Missing / Partial Operator Coverage #19412 section C: fix partial TFLite operator implementations (RANGE)
• PR [Frontend][TFLite] Add test coverage for SHAPE and RANGE operators #19401: added the RANGE dynamic-scalar guard and its test
• PR [Relax][TFLite] Introduce TensorFlow Lite frontend #18868: introduced the Relax TFLite frontend and convert_range

[gemini-code-assist]

Code Review

This pull request adds support for TFLite RANGE operators with dynamic (runtime) scalar bounds in the Relax frontend, handling both integer and float data types by computing the element count in-graph and lifting it to a symbolic dimension. It also replaces the previous unsupported-feature test with comprehensive parameterized tests. Feedback on the changes points out a critical typo where tirx is used instead of tir in the _scalar_tensor_to_dim helper, which will cause a NameError at runtime.

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