Remove hostdevice_vector::element due to unnecessary synchronization (#19092)

Contributes to #18967, part of #18968 

In this PR, `hostdevice_vector::element` is removed due to its internal `cudaMemcpy` into host pageable memory. Also, the only call in it is replaced manually.

Authors:
  - Jigao Luo (https://github.com/JigaoLuo)
  - Muhammad Haseeb (https://github.com/mhaseeb123)

Approvers:
  - Muhammad Haseeb (https://github.com/mhaseeb123)
  - Vukasin Milovanovic (https://github.com/vuule)

URL: #19092

Author: JigaoLuo

cpp/src/io/orc/writer_impl.cu

@@ -1149,7 +1149,18 @@ cudf::detail::hostdevice_vector<uint8_t> allocate_and_encode_blobs(
   // figure out the buffer size needed for protobuf format
   orc_init_statistics_buffersize(
     stats_merge_groups.device_ptr(), stat_chunks.data(), num_stat_blobs, stream);
-  auto max_blobs = stats_merge_groups.element(num_stat_blobs - 1, stream);
+
+  // get stats_merge_groups[num_stat_blobs - 1] via a host pinned bounce buffer
+  auto const max_blobs = [&]() {
+    auto max_blobs_element =
+      cudf::detail::make_pinned_vector_async<statistics_merge_group>(1, stream);
+    cudf::detail::cuda_memcpy<statistics_merge_group>(
+      max_blobs_element,
+      cudf::device_span<statistics_merge_group>{stats_merge_groups.device_ptr(num_stat_blobs - 1),
+                                                1},
+      stream);
+    return max_blobs_element.front();
+  }();
 
   cudf::detail::hostdevice_vector<uint8_t> blobs(max_blobs.start_chunk + max_blobs.num_chunks,
                                                  stream);

cpp/src/io/utilities/hostdevice_vector.hpp

@@ -100,23 +100,6 @@ class hostdevice_vector {
   [[nodiscard]] T* d_end() { return device_ptr(size()); }
   [[nodiscard]] T const* d_end() const { return device_ptr(size()); }
 
-  /**
-   * @brief Returns the specified element from device memory
-   *
-   * @note This function incurs a device to host memcpy and should be used sparingly.
-   * @note This function synchronizes `stream`.
-   *
-   * @throws rmm::out_of_range exception if `element_index >= size()`
-   *
-   * @param element_index Index of the desired element
-   * @param stream The stream on which to perform the copy
-   * @return The value of the specified element
-   */
-  [[nodiscard]] T element(std::size_t element_index, rmm::cuda_stream_view stream) const
-  {
-    return d_data.element(element_index, stream);
-  }
-
   operator cudf::host_span<T>() { return host_span<T>{h_data}.subspan(0, size()); }
   operator cudf::host_span<T const>() const
   {