Fix division by 0 when grid is full

Luca Grillottirequested to merge
github/fork/Lookatator/fix-repertoire-sample into 2-instadeep-new-structure-suggestion

Created by: Lookatator

Hi,

we noticed that when the grid was full, there was a division by 0 in the sampling function of the repertoire, which lead to inconsistencies when sampling individuals.

        grid_empty = self.fitnesses == -jnp.inf
        p = (1.0 - grid_empty) / jnp.sum(grid_empty)

This pull request aims at solving that issue.

Best regards,

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