[QTOOL-3157] Instrument halide_malloc to report allocations
[QTOOL-3157] Instrument halide_malloc to report allocations

- update load_pgm to not specify channels in ImageType constructor
- was previously specifying 1 channel for graymap but could have
  been using the constructor that does not specify any channels
- image now reported as having 2 dimensions instead of 3 since
  there is no longer a color channel dimension
- no change in performance, seen when analyzing image alignments

Change-Id: I90cf527e49c6e3674c8c776a27814aeece7ee739
Reviewed-on: https://review-hexagon.quicinc.com/46957
Reviewed-by: Ron Lieberman <4808210bfeb114024e0130f942a1d76ec9765331@quicinc.com>
Tested-by: Anshu Das Gupta <6e3c469cfefeb1dd4487310d20ec4885cd79fc50@quicinc.com>
Reviewed-by: Pranav Bhandarkar <22c0464eb03706a3368d7e4716e73a3e3072b77b@codeaurora.org>

psuriana/Halide,jiawen/Halide,ronen/Halide,psuriana/Halide,ronen/Halide,jiawen/Halide,jiawen/Halide,ronen/Halide,ronen/Halide,psuriana/Halide,kgnk/Halide,ronen/Halide,kgnk/Halide,jiawen/Halide,ronen/Halide,jiawen/Halide,kgnk/Halide,kgnk/Halide,psuriana/Halide,kgnk/Halide,kgnk/Halide,kgnk/Halide,ronen/Halide,psuriana/Halide,jiawen/Halide,psuriana/Halide,ronen/Halide,psuriana/Halide,kgnk/Halide,jiawen/Halide