Fix wrong instructions and code for custom tasks

[albertvillanova]

Fix wrong instructions and code for custom tasks.

We are giving instructions to convert custom tasks to dict but this is not necessary or done (only prints are proposed):

# Convert to dict for lighteval
if __name__ == "__main__":
    print(t.name for t in TASKS_TABLE)
    print(len(TASKS_TABLE))

In other parts of the code, we try to access attributes of Task instances as if it were a dict, which is wrong:

print(t["name"] for t in TASKS_TABLE)

• should be t.name instead

This PR removes:

• the instructions to convert to dict
• the prints

[HuggingFaceDocBuilderDev]

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[clefourrier]

Good catch! Feel free to merge once tests are OK!