[EM] Enable access to the number of batches. (#10691)

- Expose `NumBatches` in `DMatrix`.
- Small cleanup for removing legacy CUDA stream and ~force CUDA context initialization~.
- Purge old external memory data generation code.

tests/cpp/gbm/test_gbtree.cc

@@ -715,7 +715,7 @@ TEST(GBTree, InplacePredictionError) {
       p_fmat = rng.GenerateQuantileDMatrix(true);
     } else {
 #if defined(XGBOOST_USE_CUDA)
-      p_fmat = rng.GenerateDeviceDMatrix(true);
+      p_fmat = rng.Device(ctx->Device()).GenerateQuantileDMatrix(true);
 #else
       CHECK(p_fmat);
 #endif  // defined(XGBOOST_USE_CUDA)