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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def device_type_from_string(cl_device_type_str):
 """Converts values like ``gpu`` to a pyopencl device type string. Supported values are: ``accelerator``, ``cpu``, ``custom``, ``gpu``. If ``all`` is given, None is returned. Args: cl_device_type_str (str):
 The string we want to convert to a device type. Returns: cl.device_type: the pyopencl device type. """

    cl_device_type_str = cl_device_type_str.upper()
    if hasattr(cl.device_type, cl_device_type_str):
        return getattr(cl.device_type, cl_device_type_str)
    return None