Common interface for collective communication (#8057)

* implement broadcast for federated communicator

* implement allreduce

* add communicator factory

* add device adapter

* add device communicator to factory

* add rabit communicator

* add rabit communicator to the factory

* add nccl device communicator

* add synchronize to device communicator

* add back print and getprocessorname

* add python wrapper and c api

* clean up types

* fix non-gpu build

* try to fix ci

* fix std::size_t

* portable string compare ignore case

* c style size_t

* fix lint errors

* cross platform setenv

* fix memory leak

* fix lint errors

* address review feedback

* add python test for rabit communicator

* fix failing gtest

* use json to configure communicators

* fix lint error

* get rid of factories

* fix cpu build

* fix include

* fix python import

* don't export collective.py yet

* skip collective communicator pytest on windows

* add review feedback

* update documentation

* remove mpi communicator type

* fix tests

* shutdown the communicator separately

Co-authored-by: Hyunsu Cho <chohyu01@cs.washington.edu>

src/common/io.h

@@ -12,6 +12,7 @@
 #include <rabit/rabit.h>
 #include <string>
 #include <cstring>
+#include <fstream>
 
 #include "common.h"
 
@@ -111,6 +112,22 @@ inline std::string ReadAll(dmlc::Stream* fi, PeekableInStream* fp) {
   }
   return buffer;
 }
+
+/**
+ * \brief Read the whole file content into a string.
+ */
+inline std::string ReadAll(std::string const &path) {
+  std::ifstream stream(path);
+  if (!stream.is_open()) {
+    LOG(FATAL) << "Could not open file " << path;
+  }
+  std::string content{std::istreambuf_iterator<char>(stream), std::istreambuf_iterator<char>()};
+  if (content.empty()) {
+    LOG(FATAL) << "Empty file " << path;
+  }
+  return content;
+}
+
 }  // namespace common
 }  // namespace xgboost
 #endif  // XGBOOST_COMMON_IO_H_