Investigate failure when running metal delegated whisper sampler (argmax)

🐛 Describe the bug

Error reported in https://github.com/huggingface/optimum-executorch/pull/208:

E 00:00:13.059103 executorch:et_metal.mm:246] ETMetalShaderLibrary: Failed to compile shader library: program_source:3813:29: error: assigning to 'bfloat' from incompatible type 'float'
tmp_acc_2 = tmp0;
^~~~

E 00:00:13.059124 executorch:et_metal.mm:263] ETMetalShaderLibrary: Library not compiled
E 00:00:13.059126 executorch:et_metal.mm:301] ETMetalShaderLibrary::getKernelFunction: Failed to get pipeline state for 'generated_kernel'
E 00:00:13.059127 executorch:shim_mps.mm:105] aoti_torch_mps_get_kernel_function: Failed to get kernel function 'generated_kernel'
E 00:00:13.059129 executorch:shim_mps.mm:517] aoti_torch_mps_run_command_block: null function handle

This PR disabled the sampler export code path to let runner fallback to C++ CPU sampler.

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