EnglishRoberta is not thread-safe

[stephentoub]

_mergeRanks is mutated as part of encoding:

machinelearning/src/Microsoft.ML.Tokenizers/Model/EnglishRoberta.cs

Line 591 in 4635a86

var (first, second) = pairs.ArgMin(pair => _mergeRanks.GetOrAdd(pair, int.MaxValue));

[stephentoub]

cc: @tarekgh