Stack and streaming conformer support (#141)

* added csrc/stack.cc

* stack: added checks

* added copyright info

* passed cpp style checks

* formatted code

* added some support for streaming conformer model support (not verified)

* code lint

* made more progress with streaming conformer support (not working yet)

* passed style check

* changes as suggested by @csukuangfj

* added some debug info

* fixed style check

* Use Cat to replace Stack

* remove debug statements

---------

Co-authored-by: Jingzhao Ou (jou2019) <jou2019@cisco.com>
Co-authored-by: Fangjun Kuang <csukuangfj@gmail.com>

sherpa-onnx/csrc/stack.h

@@ -0,0 +1,29 @@
+// sherpa-onnx/csrc/stack.h
+//
+// Copyright (c) 2023 Jingzhao Ou (jingzhao.ou@gmail.com)
+
+#ifndef SHERPA_ONNX_CSRC_STACK_H_
+#define SHERPA_ONNX_CSRC_STACK_H_
+
+#include <vector>
+
+#include "onnxruntime_cxx_api.h"  // NOLINT
+
+namespace sherpa_onnx {
+
+/** Stack a list of tensors along the given dim.
+ *
+ * @param allocator Allocator to allocate space for the returned tensor
+ * @param values  Pointer to a list of tensors. The shape of the tensor must
+ *                be the same except on the dim to be stacked.
+ * @param dim  The dim along which to concatenate the input tensors
+ *
+ * @return Return the stacked tensor
+ */
+template <typename T = float>
+Ort::Value Stack(OrtAllocator *allocator,
+                 const std::vector<const Ort::Value *> &values, int32_t dim);
+
+}  // namespace sherpa_onnx
+
+#endif  // SHERPA_ONNX_CSRC_STACK_H_