fix: error when using graph in multi-threads.

Author: AsakusaRinne

src/TensorFlowNET.Core/Device/DeviceSpec.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Text;
 using System.Threading.Tasks;
@@ -7,8 +8,8 @@ namespace Tensorflow.Device
 {
     public class DeviceSpec
     {
-        private static Dictionary<string, Components> _STRING_TO_COMPONENTS_CACHE = new();
-        private static Dictionary<Components, string> _COMPONENTS_TO_STRING_CACHE = new();
+        private static ConcurrentDictionary<string, Components> _STRING_TO_COMPONENTS_CACHE = new();
+        private static ConcurrentDictionary<Components, string> _COMPONENTS_TO_STRING_CACHE = new();
         private string _job;
         private int _replica;
         private int _task;

test/TensorFlowNET.UnitTest/Basics/ThreadSafeTest.cs

@@ -0,0 +1,41 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+using Tensorflow;
+using static Tensorflow.Binding;
+
+namespace TensorFlowNET.UnitTest.Basics
+{
+    [TestClass]
+    public class ThreadSafeTest
+    {
+        [TestMethod]
+        public void GraphWithMultiThreads()
+        {
+            List<Thread> threads = new List<Thread>();
+
+            const int THREADS_COUNT = 5;
+
+            for (int t = 0; t < THREADS_COUNT; t++)
+            {
+                Thread thread = new Thread(() =>
+                {
+                    Graph g = new Graph();
+                    Session session = new Session(g);
+                    session.as_default();
+                    var input = tf.placeholder(tf.int32, shape: new Shape(6));
+                    var op = tf.reshape(input, new int[] { 2, 3 });
+                });
+                thread.Start();
+                threads.Add(thread);
+            }
+
+            threads.ForEach(t => t.Join());
+        }
+    }
+}