fix: Set missing env variable WORLD_SIZE temporarily for trainer init

[brb-nv]

Description

This addresses https://nvbugspro.nvidia.com/bug/5301221.

Context
We use a trainer (transformers.Trainer) while creating dummy Medusa and Eagle heads. During the trainer's init, looks like torch dist training is being called and it expects env variables such as WORLD_SIZE, MASTER_PORT, MASTER_ADDR, RANK.

To deal with a peculiar issue in transformers library, we unset WORLD_SIZE while running tests in specific cluster nodes causing this test to fail.

Proposed solution
Set WORLD_SIZE env variable temporarily at trainer init if env variable is missing.

Alternative
Skip the test when WORLD_SIZE is not set.

Test Coverage

$ pytest tests/integration/defs/examples/test_eagle.py::test_mistral_nemo_eagle_1gpu[Mistral-Nemo-12b-Base-eagle1] -s -v

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