Remove `numel` field in tensor

* It is duplicated with `dim_`. We can use `dim_` to calculate `numel`
  everytime. It does not cost too much.
* `numel` is not initialized by constructor. Also, `numel` is hard to
  synchronize with `dim_`.

So just remove it.

paddle/framework/tensor.h

@@ -165,12 +165,6 @@ class Tensor {
   /*! points to dimensions of memory block. */
   DDim dims_;
 
-  /**
-   * A cache of the number of elements in a tensor.
-   * Would be 0 for an uninitialized tensor.
-   */
-  int64_t numel_;
-
   /**
    * @brief   A PlaceHolder may be shared by more than one tensor.
    *

paddle/framework/tensor_impl.h

@@ -147,13 +147,12 @@ inline Tensor Tensor::Slice(const int& begin_idx, const int& end_idx) const {
 
 inline Tensor& Tensor::Resize(const DDim& dims) {
   dims_ = dims;
-  numel_ = product(dims_);
   return *this;
 }
 
 inline const DDim& Tensor::dims() const { return dims_; }
 
-inline int64_t Tensor::numel() const { return numel_; }
+inline int64_t Tensor::numel() const { return product(dims_); }
 
 template <typename T>
 inline Tensor ReshapeToMatrix(const Tensor& src, int num_col_dims) {