Manage runtime environments (#452)

* Add `Env` class that manages all runtime environments.
* Changed `NPKIT_DUMP_DIR` to `MSCCLPP_NPKIT_DUMP_DIR`.

include/mscclpp/env.hpp

@@ -0,0 +1,42 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+#ifndef MSCCLPP_ENV_HPP_
+#define MSCCLPP_ENV_HPP_
+
+#include <memory>
+#include <string>
+
+namespace mscclpp {
+
+class Env;
+
+/// Get the MSCCL++ environment.
+/// @return A reference to the global environment object.
+std::shared_ptr<Env> env();
+
+/// The MSCCL++ environment. The constructor reads environment variables and sets the corresponding fields.
+/// Use the @ref env() function to get the environment object.
+class Env {
+ public:
+  const std::string debug;
+  const std::string debugSubsys;
+  const std::string debugFile;
+  const std::string hcaDevices;
+  const std::string hostid;
+  const std::string socketFamily;
+  const std::string socketIfname;
+  const std::string commId;
+  const std::string executionPlanDir;
+  const std::string npkitDumpDir;
+  const bool cudaIpcUseDefaultStream;
+
+ private:
+  Env();
+
+  friend std::shared_ptr<Env> env();
+};
+
+}  // namespace mscclpp
+
+#endif  // MSCCLPP_ENV_HPP_