Fix type in decimal_avg and decimal_sum while spilling

[mskapilks]

We have been playing around with spill in partial aggregate when we noticed velox crashes when it read data back from spill in partial aggregate. This happens when input type is of ShortDecimal like decimal(12,4).
Both these agg function produces intermediate sum of LongDecimal irrespective of input. So when input is of LongDecimal it works but in case of ShortDecimal this respective dynamic type cast fails producing null pointer causing crash.

[mskapilks]

cc: @zhouyuan

[rui-mo]

cc @liujiayi771

[liujiayi771]

@rui-mo For sum agg, this probloem has been fixed in upstream PR. But for avg agg, I think the intermediate type will always be int128_t, as discussed in https://github.com/facebookincubator/velox/pull/6020/files#r1302694957, it seems that TSumResultType in this modification will always be int128_t.

[zhouyuan]

@mskapilks could you please help to review this patch? facebookincubator#6020

@rui-mo actually has a new branch based on meta/velox + several important backport patches (https://github.com/oap-project/velox/tree/update) which will become the new "main" branch soon.

thanks, -yuan