Conditionally enable GeLU approximation (#1810)

Sigmoid approximation for GeLU was introduced in #1299 for Fp16. The sigmoid approximation is known to get better perf but lower accuracy. https://arxiv.org/pdf/1606.08415.pdf

src/targets/gpu/target.cpp

@@ -74,6 +74,8 @@ namespace gpu {
 MIGRAPHX_DECLARE_ENV_VAR(MIGRAPHX_DISABLE_SCHEDULE_PASS)
 MIGRAPHX_DECLARE_ENV_VAR(MIGRAPHX_DISABLE_REDUCE_FUSION)
 MIGRAPHX_DECLARE_ENV_VAR(MIGRAPHX_ENABLE_NHWC)
+MIGRAPHX_DECLARE_ENV_VAR(MIGRAPHX_DISABLE_FAST_GELU)
+
 struct id_pass
 {
     std::string name() const { return "id"; }
@@ -121,7 +123,7 @@ std::vector<pass> target::get_passes(migraphx::context& gctx, const compile_opti
         inline_module{},
         rewrite_pooling{},
         dead_code_elimination{},
-        rewrite_gelu{},
+        enable_pass(not enabled(MIGRAPHX_DISABLE_FAST_GELU{}), rewrite_gelu{}),
         optimize_module{},
         enable_pass(enabled(MIGRAPHX_ENABLE_NHWC{}), layout_nhwc{}),
         dead_code_elimination{},