[te] Fix pow (pytorch#48213)

Summary:
Pull Request resolved: pytorch#48213

it was completely broken unless rhs was a constant.

Test Plan: new unit test in test_jit_fuser_te.py

Reviewed By: eellison

Differential Revision: D25071639

fbshipit-source-id: ef1010a9fd551db646b83adfaa961648a5c388ae

Author: bertmaher

test/test_jit_fuser_te.py

@@ -1366,6 +1366,25 @@ def eager(a, b):
         b = torch.randint(-2, 2, (1, 64), device='cuda', dtype=torch.long)
         script = self.checkScript(eager, (a, b))
 
+    def test_neg_pow(self):
+        def eager_tt(a: torch.Tensor, b: torch.Tensor):
+            return torch.neg(torch.pow(a, b))
+
+        def eager_ts(a: torch.Tensor, b: float):
+            return torch.neg(torch.pow(a, b))
+
+        def eager_st(a: float, b: torch.Tensor):
+            return torch.neg(torch.pow(a, b))
+
+        a = torch.rand(1, dtype=torch.float)
+        b = torch.rand(1, dtype=torch.float)
+        s = b.item()
+        script = self.checkScript(eager_tt, (a, b))
+        self.assertAllFused(script.graph_for(a, b))
+        script = self.checkScript(eager_ts, (a, s))
+        self.assertAllFused(script.graph_for(a, s))
+        script = self.checkScript(eager_st, (s, b))
+        self.assertAllFused(script.graph_for(s, b))
 
 if __name__ == '__main__':
     run_tests()

torch/csrc/jit/tensorexpr/kernel.cpp

@@ -1026,10 +1026,11 @@ Tensor* TensorExprKernel::computeValue(const torch::jit::Value* v) {
     case aten::pow: {
       return computeTwoOperand(
           "aten_pow", v, [](const ExprHandle& lhs, const ExprHandle& rhs) {
-            double val = 0;
-            if (rhs.node()->isConstant()) {
-              val = immediateAs<double>(IRSimplifier::simplify(rhs.node()));
+            if (!rhs.node()->isConstant()) {
+              return pow(lhs, rhs);
             }
+            double val =
+                immediateAs<double>(IRSimplifier::simplify(rhs.node()));
 
             if (val == 1.0f) {
               return lhs;