Adds missing license headers.

[codemzs]

In addition to #3645, I wrote a small C# program to open every ".cs" file in src directory and read the first non-null/empty line and compare it against "// Licensed to the .NET Foundation under one or more agreements." and found few more places where license was missing.

public static class Program
    {
        public static void Main(string[] args) => RunAll(@"E:\machinelearning\src");

        internal static void RunAll(string sDir)
        {
           foreach (string d in Directory.GetDirectories(sDir))
            {
                foreach (string f in Directory.GetFiles(d))
                {
                    if (Path.GetExtension(f) == ".cs" || Path.GetExtension(f) == ".tt")
                    {
                        using (var fileStream = File.OpenRead(f))
                        using (var streamReader = new StreamReader(fileStream, Encoding.UTF8, true))
                        {
                            string line;
                            while ((line = streamReader.ReadLine()) != null)
                            {
                                if(String.IsNullOrEmpty(line))
                                    continue;

                                if (line !=
                                    @"// Licensed to the .NET Foundation under one or more agreements.")
                                {
                                    Console.WriteLine(f + " : " + line);
                                }

                                break;

                            }
                        }
                    }
                }
                RunAll(d);
            }
        }
    }

[codecov]

Codecov Report

Merging #3651 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3651   +/-   ##
=======================================
  Coverage   72.78%   72.78%           
=======================================
  Files         808      808           
  Lines      145588   145588           
  Branches    16250    16250           
=======================================
  Hits       105960   105960           
- Misses      35204    35205    +1     
+ Partials     4424     4423    -1

Flag	Coverage Δ
#Debug	72.78% <ø> (ø)	⬆️
#production	68.28% <ø> (ø)	⬆️
#test	89.03% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
...c/Microsoft.ML.FastTree/Utils/BufferPoolManager.cs	0% <ø> (ø)	⬆️
...icrosoft.ML.TensorFlow/TensorFlow/TensorGeneric.cs	44.21% <ø> (ø)	⬆️
src/Microsoft.ML.Transforms/Text/LdaTransform.cs	89.26% <0%> (-0.63%)	⬇️
...StandardTrainers/Standard/LinearModelParameters.cs	60.31% <0%> (+0.26%)	⬆️
src/Microsoft.ML.Maml/MAML.cs	26.21% <0%> (+1.45%)	⬆️

[ganik]