Small fixups for enabling zero size dims. (#9724)

Summary:
1) Properly test cpu for alpha/beta addmm cases.
2) Unsqueeze on empty no longer throws an exception.
Pull Request resolved: pytorch/pytorch#9724

Reviewed By: ezyang

Differential Revision: D8958513

Pulled By: gchanan

fbshipit-source-id: 6ce2ec4a47201f9b225b8c52354144ace43e9e09

Author: gchanan

aten/src/TH/generic/THTensorMath.cpp

@@ -859,6 +859,16 @@ void THTensor_(addmv)(THTensor *r_, real beta, THTensor *t, real alpha, THTensor
     THTensor_(free)(cmat);
   }
 
+  // In gemv (x,0).mv(0) does not
+  // handle beta, whereas gemm does for case where (x,0).mm(0,y).
+  if (vec->size(0) == 0 && mat->size(0) != 0) {
+    if (beta == 0) {
+      THTensor_(zero)(r_);
+    } else if (beta != 1) {
+      THTensor_(mul)(r_, r_, beta);
+    }
+  }
+
   #undef LDA_COND
 }
 

aten/src/THC/generic/THCTensorMathBlas.cu

@@ -116,7 +116,7 @@ THCTensor_(addmv)(THCState *state, THCTensor *r_, real beta, THCTensor *t, real
     THCTensor_(free)(state, cmat);
   }
 
-  // cublasSgemv, cublasDgemv have a bug where (x,0).mv(0) does not
+  // In cublasSgemv, cublasDgemv (x,0).mv(0) does not
   // handle beta, whereas cublasSgemm, cublasDgemm do for case where (x,0).mm(0,y).
   if (vec->size(0) == 0 && mat->size(0) != 0) {
     if(THCNumerics<real>::eq(beta, ScalarConvert<int, real>::to(0))) {