code: def _unsqueeze(a, axis):  # type: ignore
      try:
          return np.expand_dims(a, axis=axis)
      except TypeError:
          # numpy 1.18 supports axes as a tuple
          if len(axis) == 1:
              return np.expand_dims(a, axis=tuple(axis)[0])
          for x in reversed(axis):
              a = np.expand_dims(a, axis=x)
          return a
  
file_path: onnx/onnx/reference/ops/op_stft.py
imports: import numpy
  from onnx.reference.op_run import OpRun
  from onnx.reference.ops.op_concat_from_sequence import _concat_from_sequence
  from onnx.reference.ops.op_dft import _cfft
  from onnx.reference.ops.op_slice import _slice
label_desc: Use image node_id 2 for calling a global function with example usage: _unsqueeze(a, axis) and returns: np, a, np