Changed Dictionary to ConcurrentDictionary

[harishsk]

We are seeing new test failure sporadically with the following call stack:

[xUnit.net 00:00:05.18]     Microsoft.ML.Functional.Tests.Debugging.ViewTrainingOutput [FAIL]

  X Microsoft.ML.Functional.Tests.Debugging.ViewTrainingOutput [16ms]
  Error Message:
   System.InvalidOperationException : Operations that change non-concurrent collections must have exclusive access. A concurrent update was performed on this collection and corrupted its state. The collection's state is no longer correct.
  Stack Trace:
     at System.Collections.Generic.Dictionary`2.FindEntry(TKey key)
   at System.Collections.Generic.Dictionary`2.ContainsKey(TKey key)
   at Microsoft.ML.Functional.Tests.Debugging.LogWatcher.ObserveEvent(Object sender, LoggingEventArgs e) in /Users/runner/runners/2.166.3/work/1/s/test/Microsoft.ML.Functional.Tests/Debugging.cs:line 204
   at Microsoft.ML.MLContext.ProcessMessage(IMessageSource source, ChannelMessage message) in /Users/runner/runners/2.166.3/work/1/s/src/Microsoft.ML.Data/MLContext.cs:line 135
   at Microsoft.ML.Runtime.HostEnvironmentBase`1.Dispatcher`1.DispatchCore(IMessageSource sender, TMessage message) in /Users/runner/runners/2.166.3/work/1/s/src/Microsoft.ML.Core/Environment/HostEnvironmentBase.cs:line 314

Since the dictionary is being modified by multiple threads, I am changing this to a ConcurrentDictionary.

[frank-dong-ms-zz]

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@b74cacc). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #5097   +/-   ##
=========================================
  Coverage          ?   75.61%           
=========================================
  Files             ?      993           
  Lines             ?   178514           
  Branches          ?    19191           
=========================================
  Hits              ?   134987           
  Misses            ?    38298           
  Partials          ?     5229           

Flag	Coverage Δ
#Debug	75.61% <100.00%> (?)
#production	71.57% <ø> (?)
#test	88.64% <100.00%> (?)

Impacted Files	Coverage Δ
test/Microsoft.ML.Functional.Tests/Debugging.cs	100.00% <100.00%> (ø)