[Fix][Relax] Dequantize fp8 as a float, not an integer

[andrewfish0]

relax.dequantize truncates fp8 inputs to integers before the scale is applied.

dequantize_compute picks its intermediate dtype with matches_code(FLOAT, BFLOAT) — DLPack codes 2 and 4. Every narrow float carries its own code instead (Float8E3M4 at 7 through Float4E2M1FN at 17), so an fp8 input takes the int32 branch:

# before
dequantized[v_i0, v_i1] = T.Cast("float32", T.Cast("float16", T.Cast("int32", data[v_i0, v_i1])) - zp[v_i0]) * scale[v_i0]

# after
dequantized[v_i0, v_i1] = (T.Cast("float32", data[v_i0, v_i1]) - T.Cast("float32", zp[v_i0])) * scale[v_i0]

A stored -0.9375 becomes 0.

• Reachable through the public API: relax.quantize already accepts float8_e4m3fn and float8_e5m2 as output dtypes.
• Easy to miss: the truncation is a no-op wherever the stored value is already integral, which at E4M3's spacing covers most of the top of the range.
• quantize is unaffected — it decides the same question with "int" in out_dtype, which excludes the fp8 dtypes correctly.

Change

• Add PrimType.is_float, forwarding to the is_float that tvm_ffi already provides, so the float code list keeps a single home and a future narrow float needs no edit here.
• Use it in _dequantize, and drop the now-unused DataTypeCode import.

Tests

• test_dequantize_float8_e4m3fn_to_fp32 in test_transform_legalize_ops_qdq.py. Every existing dequantize case in that file starts from int8, and the fp8 coverage in test_op_qdq.py stops at type inference, so nothing checked the TIR generated for an fp8 input.
• Confirmed it fails without the fix (structural mismatch on the int32 cast) and passes with it. 277 passed, 3 skipped across test_transform_legalize_ops*.py.

Not covered

• The same matches_code(FLOAT, BFLOAT) pattern appears at six other Python sites, including legalize_ops/common.py:83, where an fp8 scalar constant would fall through both branches. I have not assessed whether narrow floats reach any of them, and have left them alone.

[andrewfish0]

@tqchen @hunghsiangwang can you review?