Various bug fixes (#2825)

* Fatal error if GPU algorithm selected without GPU support compiled

* Resolve type conversion warnings

* Fix gpu unit test failure

* Fix compressed iterator edge case

* Fix python unit test failures due to flake8 update on pip

src/data/sparse_page_source.cc

@@ -120,7 +120,7 @@ void SparsePageSource::Create(dmlc::Parser<uint32_t>* src,
     double tstart = dmlc::GetTime();
     // print every 4 sec.
     const double kStep = 4.0;
-    size_t tick_expected = kStep;
+    size_t tick_expected = static_cast<double>(kStep);
 
     while (src->Next()) {
       const dmlc::RowBlock<uint32_t>& batch = src->Value();
@@ -149,7 +149,7 @@ void SparsePageSource::Create(dmlc::Parser<uint32_t>* src,
           LOG(CONSOLE) << "Writing row.page to " << cache_info << " in "
                        << ((bytes_write >> 20UL) / tdiff) << " MB/s, "
                        << (bytes_write >> 20UL) << " written";
-          tick_expected += kStep;
+          tick_expected += static_cast<size_t>(kStep);
         }
       }
     }