Halve import time by removing torch dependency

[aymeric-roucher]

Issue raised by @xianminx: #100

This PR removes torch dependency to make install and imports faster.

Tested on this script:

import time

t0 = time.time()
from smolagents import CodeAgent, HfApiModel
t1 = time.time()

print(f"Total time: {t1 - t0:.2f}")

The runtime falls from 6.52s to 3.8s, nearly halved!

[HuggingFaceDocBuilderDev]

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