[TIR][CUDA] Preserve float precision in codegen with hexfloat output

[LeiWang1999]

Previously, float constants in codegen were always emitted in scientific decimal format, e.g.:

bfloat16_t(3.487723e-05f);

This could introduce slight rounding differences compared to the actual binary representation, since the constant is printed and then re-parsed in decimal. we now emit the value in hexadecimal floating-point format (std::hexfloat) to preserve the exact binary value, and additionally include the decimal form as a comment for readability:

bfloat16_t(0x1.2492492492492p-15f /*3.487723e-05*/)

[tqchen]

need to update the testcase and we are good to go

[LeiWang1999]

need to update the testcase and we are good to go

added a test case at tests/python/codegen/test_target_codegen_cuda.py: https://github.com/apache/tvm/pull/18320/files#diff-de74af0a4254367528294a58410a1c2488d014d3fafdbd8446f88b621358ad4dR804-R820

[tqchen]

i meant that we should fix https://ci.tlcpack.ai/blue/organizations/jenkins/tvm-gpu/detail/PR-18320/2/pipeline which likely is caused by expected code in the original form