[jira] [Created] (SINGA-205) Enable slice and concatenate operations for Tensor objects

["wangwei (JIRA)" <ji...@apache.org>]

wangwei created SINGA-205:
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             Summary: Enable slice and concatenate operations for Tensor objects
                 Key: SINGA-205
                 URL: https://issues.apache.org/jira/browse/SINGA-205
             Project: Singa
          Issue Type: New Feature
            Reporter: wangwei
            Assignee: wangwei


Slice and concatenate operations are typical array operations, and are necessary to support some models, e.g., GoogleLeNet.
For this ticket, we are going to implement the two operations to support 1D and 2D tensors.  The following functions would be implemented,
1. SliceRows(Tensor, start, end), the returned tensor shares the memory with the origin tensor.
2. CopyRows(Tensor, start, end), CopyColumns(Tensor, start, end) the returned tensor copies values from the original tensor
3. ConcatenateRows(vector<Tensor>) and ConcatenateColumns(vector<Tensor>); the returned tensor copies values from the origin tensors.

To support SliceRows, i.e., memory sharing between two tensors of different sizes, we need to update the Block class as

{code}
class Block {
  Block(void* ptr, size_t size, size_t offset=0);
  void* mutable_data() const  { return data_+offset_;}
  const void* data() const  { return data_+offset_;}
  private:
    void *data_;
    size_t offset_ = 0;  // offset in terms of Bytes.
    shared_ptr<atomic<int>> ref_count_;
};
{code}
The sub-tensors (i.e., generated from SliceRows) would have a different Block instance as the original tensor, but would share the internal `ref_count_` and `data_` fields.
`offset_` is 0 for the original tensors, but non-zero for sub-tensors . atomic makes the inc and dec operations thread-safe. shared_ptr helps to free `ref_count_` safely.


SliceColumns is not supported as singa::Tensor uses row major storage which is difficult to support SliceColumns SliceRows(). Users can use CopyColumns().




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