Add ConcurrentLoader (#7512)

Works just like the GenericLoader but concurrently for those who choose
to optimize their workflow.

@rlancemartin @eyurtsev

---------

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

docs/extras/integrations/document_loaders/concurrent.ipynb

@@ -0,0 +1,94 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "id": "23c6e167",
+   "metadata": {},
+   "source": [
+    "# Concurrent Loader\n",
+    "\n",
+    "Works just like the GenericLoader but concurrently for those who choose to optimize their workflow.\n"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "id": "6ff3fb1f",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "from langchain.document_loaders import ConcurrentLoader"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 8,
+   "id": "ce96fa20",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "loader = ConcurrentLoader.from_filesystem('example_data/', glob=\"**/*.txt\")"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 9,
+   "id": "06a6cf5d",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "files = loader.load()"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 12,
+   "id": "b87d3e58",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "2"
+      ]
+     },
+     "execution_count": 12,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "len(files)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "668f1ee5",
+   "metadata": {},
+   "outputs": [],
+   "source": []
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3 (ipykernel)",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.9.1"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 5
+}