Fix AllReduce3 auto-tune launch parameters

[sugarlxn]

Summary

This PR fixes the launch parameters used by the AllReduce3 auto-tune path.

Previously, the kernel launch used hardcoded values:

self.kernel.launch_kernel(self.params, 24, 1024, 0, stream)

This prevented the auto-tuned nblocks and block_size values from actually being applied during execution. The launch now uses the configured parameters:

self.kernel.launch_kernel(self.params, self.nblocks, self.block_size, 0, stream)

Changes

• Replace hardcoded kernel launch parameters with self.nblocks and self.block_size.
• Ensure AllReduce3 uses the parameters selected by auto-tuning.
• Make launch behavior consistent with the configurable tuning interface.

Motivation

The auto-tune logic should be able to control the kernel launch configuration. Using fixed values makes tuning ineffective and may lead to suboptimal performance across different message sizes, GPU counts, or hardware environments.

Testing

• Verified that the kernel launch now uses the tuned nblocks and block_size values.
• Confirmed that the updated launch path can still be automatically merged.

[sugarlxn]

Hi maintainers, could someone please review this PR and approve the pending workflows when convenient?

This PR fixes the AllReduce3 auto-tune path by replacing the hardcoded kernel launch parameters with the configured self.nblocks and self.block_size, so the tuned launch configuration can actually take effect.

Thanks!