[Convert] Tested and fixed with videotestsrc

[Convert] Tested and fixed with videotestsrc

This is not an issue. Don't be alarmed.
- Regardless of videotestsrc framerate, convert2tensor gives ~1000 FPS outputs.
  . This happens when a sink-element attached at the src-pad of this element is not synching. Add sync option to the sink element.

How-to-test:
$ gst-launch-1.0 --gst-plugin-path=/PATH_TO_BUILT_SO videotestsrc ! video/x-raw,width=280,height=40,framerate=1/1 ! convert2tensor ! filesink location="test.log" sync=true

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>