Fix Tensor::Slice with dims[0] == 1.

paddle/framework/tensor_impl.h

@@ -130,7 +130,10 @@ inline Tensor Tensor::Slice(const int& begin_idx, const int& end_idx) const {
   PADDLE_ENFORCE_LE(end_idx, dims_[0], "Slice end index is out of bound.");
   PADDLE_ENFORCE_LT(begin_idx, end_idx,
                     "Begin index must be less than end index.");
-  PADDLE_ENFORCE_NE(dims_[0], 1, "Can not slice a tensor with dims_[0] = 1.");
+
+  if (dims_[0] == 1) {
+    return *this;
+  } else {
     size_t base = numel() / dims_[0];
     Tensor dst;
     dst.holder_ = holder_;
@@ -139,6 +142,7 @@ inline Tensor Tensor::Slice(const int& begin_idx, const int& end_idx) const {
     dst.Resize(dst_dims);
     dst.offset_ = offset_ + begin_idx * base * sizeof(T);
     return dst;
+  }
 }
 
 inline Tensor& Tensor::Resize(const DDim& dims) {