Initial export of Sonnet 2.

Sonnet 2 is a re-write of Sonnet for TensorFlow 2.0, it is built on top of
`tf.Module` enabling a simple and researcher friendly interface to TensorFlow.

Please note that Sonnet 2 is currently alpha. We would love to have you use it
as an early adopter, but please make sure to test that the built in modules
behave as you expect. We would love your feedback and read all the issues posted
on GitHub.

```

                       2S2
                     22SSSS22
                  22S2SSSSSSSS22
               22S2SSS2    SSSSSS22
             SS2SSSSS        2SSSSSS2
               2SSSSSS22        2SSSSSS2
      2S22        SSS2SSS2S        2SS2SSS
     2SSSSSS2        SSSS2SS22        SSSSS2
        SSSS2SSS        SSSSSSSSS            S2
    2222   SSSSSSSS        SSS2SSSS2      2S2S2
    222222S   SSSSSSS2      2SSS2SS2   S2S2S22S
    2222222222   SSSSSS22S2S2S2S2   S22S22S2
    222S S22222S2   SS2SSS2SS2   S22S2S2
    2222    2222222S   SSS2   S2S2S2S        S2
    2222       S222222     SS2S2222        S2S2
    22222         222222  2S2222        S2S2S22
    SS222222         222  2S22       S222S2S222
       2222222S           2S22     22S2S2222S22
          S2222222        2S22  2S2S2222  2S2S2
    S        S222222S     2S2S2S22S22     2S222
    22S         S222222S  2S22S2S2        22S22
    222222         S2222  2S2S2         22S2S22
    22222222S       S222  22S        22S2S2S
       2S222222S    S222          22S2S2SS
          222222222 S222       22S2S2SS
             2S222222222    222S22SS2
                2S222222  2S2S22SS
                   2S222  22SS2
                      2S  22
```

c.f.: google-deepmind#117

PiperOrigin-RevId: 247390692