Count dataset tokens

[plaguss]

Description

This PR adds a way of computing summary statistics of the distiset, and adds a table in the final README template card.
It computes the statistics per leaf node and shows a table per each one like the following:

---

Dataset Statistics

• Summary statistics: default

	mean	std	min	max	sum
input_tokens_statistics_generation	1881	1.41421	1880	1882	3762
output_tokens_statistics_generation	582	1.41421	581	583	1164

---

Should be merged after #1034

Closes #1046

[github-actions]

Documentation for this PR has been built. You can view it at: https://distilabel.argilla.io/pr-1055/

[codspeed-hq]

CodSpeed Performance Report

Merging #1055 will not alter performance

_{Comparing count-dataset-tokens (5741cd1) with develop (e830e25)}

Summary

✅ 1 untouched benchmarks