[FEA] Output keyset file

[albert17]

Generate and output file in binary file, with the unique keys of all the cat features in sequential order.

@yingcanw @jershi425 Please, add more details.

@oyilmaz-nvidia for viz.

[jershi425]

We need a binary file which contains all of unique keys of all the categorical features in sequential order.

Some clarifications:

1. unique keys of all the categorical features mean that there should NOT be any duplicates, even across features.
2. The file size should be exactly the size(4 bytes or 8 bytes) of key type(int32 or int64) * number of unique keys. In another word, we don't need any separators in the file.
3. The naming of file is *.keyset
4. The number of unique keys is also equal to the sum of embedding sizes(can be generated using the get_embedding_sizes method in NVT).

For example:
Suppose we have "feature1":[key1, key2, key3], "feature2:"[key1, key2], "feature3:"[key1, key2, key3, key4].
The embedding sizes of feature1, feature2, feature3 are 3, 2, 4, respectively. So the total number of unique keys is equal to 3 + 2 +4 = 9.
Therefore, what in the .keyset file should be 123456789 (in binary format).