[ARITH] cleanup the indexmod/div on python side

[tqchen]

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[tqchen]

@kevinthesun @icemelon9 please check the changes in the multi-box prior. The error was detected when performing an integer division that has ambiguity, but seems we should have do float div.

[kevinthesun]

@kevinthesun @icemelon9 please check the changes in the multi-box prior. The error was detected when performing an integer division that has ambiguity, but seems we should have do float div.

@tqchen Do you mean we need to explicitly cast size[i] * height to avoid such error?

[tqchen]

just to confirm what is the intend of the code. I changed the code to explicitly cast to float. Otherwise it will do integer division(the result will be rounded)

[tqchen]

@kevinthesun to elaborate the further, previously the code was

ratio_int * height / width / 2.0 

Originally because both ratio_int * height and width have int type, the division will be translated to integer division(which truncates). Imagine ratio_int = 2, height=10, width = 20, the result of ratio_int * height / width will be 2 * 10 / 20 = 1.

The current set of change detects this division and raises an error, so we can explicit do the cast which will give the floating pt division.

float32(ratio_int * height) / width

I assumed that the floating pt division is what we want here and we just revealed a previous bug in the code. but it would be great if you can also help confirm that also cc @icemelon9