Add README to each directory if it contains multiple examples

[nzw0301]

Motivation

Some directories provide multiple different examples. For instance, pytorch ditrctory propvides multiple settings with pure pytorch and pytorch's wrapper examples. Another example is terminator example. Since directories are also linked from optuna/optuna directly, the top README.md is not enough to give clear information of the directory.

Description

Add README for some complicated directory and write a short description of the usage or purpose of example files.

Alternatives (optional)

Additional context (optional)

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