add rmsnorm CK_TILE_FLOAT_TO_BFLOAT16_DEFAULT compile config

[zhyajie]

Summary

Add compile-time configuration for FP32 to BF16 rounding mode in RMSNorm CK kernel.

This PR allows users to control the FP32 to BF16 conversion behavior in CK RMSNorm kernel via the CK_TILE_FLOAT_TO_BFLOAT16_DEFAULT environment variable.

Changes

• Add -DCK_TILE_FLOAT_TO_BFLOAT16_DEFAULT compile flag to module_rmsnorm in optCompilerConfig.json
• Default value is 2 (Truncate mode, same as CK default)

Rounding Mode Options

Value	Mode	Description
0	STANDARD	RNE (Round to Nearest Even) - software implementation
1	TRUNCATE_WITH_NAN	Truncate with NaN preservation
2	TRUNCATE	Fast truncate (default)
3	STANDARD_ASM	RNE - optimized asm implementation
4	RTA_ASM	Round to Nearest Away - asm implementation

Usage

To use RNE (Round to Nearest Even) rounding mode:

export CK_TILE_FLOAT_TO_BFLOAT16_DEFAULT=3
# rebuild aiter

Motivation

Different rounding modes may affect numerical precision in model inference. RNE is the IEEE 754 default and provides better numerical stability for precision-sensitive workloads.