Disable invalid check for completely sparse batch that results in failed assertion for issue #1827 (#2213)

src/data/simple_csr_source.cc

@@ -41,7 +41,12 @@ void SimpleCSRSource::CopyFrom(dmlc::Parser<uint32_t>* parser) {
     if (batch.weight != nullptr) {
       info.weights.insert(info.weights.end(), batch.weight, batch.weight + batch.size);
     }
-    CHECK(batch.index != nullptr);
+    // Remove the assertion on batch.index, which can be null in the case that the data in this
+    // batch is entirely sparse. Although it's true that this indicates a likely issue with the
+    // user's data workflows, passing XGBoost entirely sparse data should not cause it to fail.
+    // See https://github.com/dmlc/xgboost/issues/1827 for complete detail.
+    // CHECK(batch.index != nullptr);
+
     // update information
     this->info.num_row += batch.size;
     // copy the data over