Robert McNamara said that Castro told him, personally and many years later, that he did explicitly advocate for the use of nuclear weapons. But he was specifically talking about the use of tactical nuclear weapons — lower yield weapons aimed at troops — and only in the instance of a US invasion of Cuba, and this came in the context of the Cuban Missile Crisis where it seemed very likely that the US would invade. McNamara quotes Castro:

Now, we started from the assumption that if there was an invasion of Cuba, nuclear war would erupt. We were certain of that ... we would be forced to pay the price, that we would disappear.... Would I have been ready to use nuclear weapons? Yes, I would have agreed to the use of nuclear weapons. ... I would have agreed, in the event of the invasion you are talking about, with the use of tactical nuclear weapons.... If Mr. McNamara or Mr. Kennedy had been in our place, and had their country been invaded, or their country was going to be occupied ... I believe they would have used tactical nuclear weapons.

McNamara tells this story in several works, including the film Fog of War, but this quote comes from here. In Fog of War he clarifies that Castro knew Cuba would be destroyed in such a scenario.

During the Crisis itself, Castro sent Khrushchev a telegram, on October 26, which said, among other things:

If ... the imperialists attack Cuba with the aim of occupying it, then the danger posed by such an aggressive measure will be so immense for all humanity that the Soviet Union will in circumstances be able to allow it, or to permit the creation of conditions in which the imperialists might initiate a nuclear strike against the USSR as well.

I say this because I believe that the aggressiveness of the imperialists is becoming extremely dangerous.

If they initiate an attack on Cuba — a barbaric, illegal, and amoral act — then in those circumstances the moment would be right for considering the elimination of such a danger, claiming the lawful right to self-defense. However difficult and horrifying this decision may be, there is, I believe, no other recourse. This opinion of mine has been formed by the emergence of an aggressive policy in which the imperialists ignore not only public opinion but all principles and rights as well: they blockade the sea, they violate air space, they are preparing an attack, and moreover they are destroying all possibilities for negotiations, even though they are aware of the gravity of the consequences.

Which is a further wrinkle: it is Castro saying, in effect, "if they invade us, you should nuke them — even if they don't use nukes — because it's too dangerous to let them survive at that point." Is that irrational? If you think Castro is concerned only about self-preservation, maybe. But he is appealing to higher ideals (in the same way that the "better dead than Red" people in the US did), and he was also (in this near-midnight telegram sent in the middle of the Crisis) likely in a pretty extreme emotional state, believing the end of his nation in the next 3 days to be "almost inevitable." But again, even this framing is essentially "defensive." It's advocating a first use of nukes, but only in situation in which the US has, in Castro's eyes, done something that would already cause existential risk to skyrocket.

Even after the Crisis had ended, Castro fought to try and keep Soviet tactical nuclear weapons on the island, to serve as a deterrent to invasion, as he did not take seriously Kennedy's commitment to avoid further aggression towards Cuba. The Soviets opted not to do this, to Castro's frustration and anger. (This is discussion in Sergei Mikoyan, The Soviet Cuban Missile Crisis, 223-227.)

Now, it is important to see that this is not about Fidel Castro wanting to launch nuclear weapons for the heck of it. This is a positive desire to use nuclear weapons — even with the expectation of self-destruction — if the US was going down a path that made Castro think that self-destruction was inevitable anyway. So this is perhaps different from what your original statement implied, which is more of a "crazy president" scenario than a "use them to repel an invasion" scenario.