[ExecuTorch] Dramatically improve op_clamp build time (2/2)

Instead of building `O(|CTYPE_IN| * |CTYPE_MIN| * |CTYPE_MAX|
* |CTYPE_OUT|)` kernel code (where |T| means the number of
possibilities for type T), we build `O((|CTYPE_IN| + |CTYPE_MIN| +
|CTYPE_MAX|) * |CTYPE_OUT|)` kernel code. (Concretely,
`ET_SWITCH_REALHB_TYPES` has 9 possibilities, so I estimate that we
went from 9**4 = 6561 template instantiations to 9 * 3 * 9 = 243
kernels, or a 27x reduction.)

Differential Revision: [D63681034](https://our.internmc.facebook.com/intern/diff/D63681034/)

ghstack-source-id: 245613707
Pull Request resolved: #5784

Author: swolchok

kernels/portable/cpu/op_clamp.cpp

@@ -214,43 +214,51 @@ Tensor& clamp_tensor_out(
 
   constexpr auto name = "clamp.Tensor_out";
 
-  ET_SWITCH_REALHB_TYPES(in_type, ctx, name, CTYPE_IN, [&]() {
+  ET_SWITCH_REALHB_TYPES(out_type, ctx, name, CTYPE_OUT, [&]() {
+    using ToCtypeOutFn = CTYPE_OUT (*)(const void*);
+    ToCtypeOutFn in_to_out;
+    ET_SWITCH_REALHB_TYPES(in_type, ctx, name, CTYPE_IN, [&]() {
+      in_to_out = [](const void* inPtr) {
+        return static_cast<CTYPE_OUT>(
+            *reinterpret_cast<const CTYPE_IN*>(inPtr));
+      };
+    });
+    ToCtypeOutFn min_to_out;
     ET_SWITCH_REALHB_TYPES(min_type, ctx, name, CTYPE_MIN, [&]() {
-      ET_SWITCH_REALHB_TYPES(max_type, ctx, name, CTYPE_MAX, [&]() {
-        ET_SWITCH_REALHB_TYPES(out_type, ctx, name, CTYPE_OUT, [&]() {
-          apply_ternary_elementwise_fn<CTYPE_OUT>(
-              [has_min, has_max](
-                  const CTYPE_OUT val_in,
-                  const CTYPE_OUT val_min,
-                  const CTYPE_OUT val_max) {
-                CTYPE_OUT val_out = val_in;
-                if (has_min) {
-                  val_out = utils::max_override(val_out, val_min);
-                }
-                if (has_max) {
-                  val_out = utils::min_override(val_out, val_max);
-                }
-                return val_out;
-              },
-              in,
-              min,
-              max,
-              out,
-              [](const void* inPtr) {
-                return static_cast<CTYPE_OUT>(
-                    *reinterpret_cast<const CTYPE_IN*>(inPtr));
-              },
-              [](const void* minPtr) {
-                return static_cast<CTYPE_OUT>(
-                    *reinterpret_cast<const CTYPE_MIN*>(minPtr));
-              },
-              [](const void* maxPtr) {
-                return static_cast<CTYPE_OUT>(
-                    *reinterpret_cast<const CTYPE_MAX*>(maxPtr));
-              });
-        });
-      });
+      min_to_out = [](const void* minPtr) {
+        return static_cast<CTYPE_OUT>(
+            *reinterpret_cast<const CTYPE_MIN*>(minPtr));
+      };
     });
+    ToCtypeOutFn max_to_out;
+    ET_SWITCH_REALHB_TYPES(max_type, ctx, name, CTYPE_MAX, [&]() {
+      max_to_out = [](const void* maxPtr) {
+        return static_cast<CTYPE_OUT>(
+            *reinterpret_cast<const CTYPE_MAX*>(maxPtr));
+      };
+    });
+
+    apply_ternary_elementwise_fn<CTYPE_OUT>(
+        [has_min, has_max](
+            const CTYPE_OUT val_in,
+            const CTYPE_OUT val_min,
+            const CTYPE_OUT val_max) {
+          CTYPE_OUT val_out = val_in;
+          if (has_min) {
+            val_out = utils::max_override(val_out, val_min);
+          }
+          if (has_max) {
+            val_out = utils::min_override(val_out, val_max);
+          }
+          return val_out;
+        },
+        in,
+        min,
+        max,
+        out,
+        in_to_out,
+        min_to_out,
+        max_to_out);
   });
 
   return out;