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/common/quantile.h

@@ -680,12 +680,12 @@ class QuantileSketchTemplate {
     nlevel = 1;
     while (true) {
       limit_size = static_cast<size_t>(ceil(nlevel / eps)) + 1;
-      size_t n = (1UL << nlevel);
+      size_t n = (1ULL << nlevel);
       if (n * limit_size >= maxn) break;
       ++nlevel;
     }
     // check invariant
-    size_t n = (1UL << nlevel);
+    size_t n = (1ULL << nlevel);
     CHECK(n * limit_size >= maxn) << "invalid init parameter";
     CHECK(nlevel <= limit_size * eps) << "invalid init parameter";
     // lazy reserve the space, if there is only one value, no need to allocate space