SketchBatchNumElements minor bugs

[hliuca]

1. In xgboost/blob/master/src/common/hist_util.cu, SketchBatchNumElements,

   auto avail = dh::AvailableMemory(device) * 0.8;
   this is a double, and this function returns size_t.
   return value, return std::min(sketch_batch_num_elements, kIntMax);

2. In xgboost/blob/master/tests/cpp/common/test_hist_util.cu, TEST(HistUtil, SketchBatchNumElements),

   auto avail = static_cast<size_t>(dh::AvailableMemory(device) * 0.8);
   this is correct, not double. However, "avail_elem" isn't compared with KIntMax, which can be a very large integer. It causes the following code fails,

   "ASSERT_EQ(batch, avail_elem);"

Thanks.

[trivialfis]

Thank you for pointing it out. The heuristic for restricting the size of sketching is not particularly robust and we have been struggling to find an alternative.

[ashutoshk314]

Hello, I would like to work on this issue and submit a fix.