add from_pretrained

[A9isha]

Description

Adding a simplified API of from_pretrained() which can work minimally as

import maxtext as mt
model,mesh, config = mt.from_pretrained(model_name=“Qwen3-0.8b”)

and also

import maxtext as mt
argv =    ["", 
            f"{MAXTEXT_CONFIGS_DIR}/base.yml", 
            f"load_parameters_path={MODEL_CHECKPOINT_PATH}/0/items",
            f"model_name={MODEL_NAME}",
         ]
model,mesh, config = mt.from_pretrained(argv)

as well as

import maxtext as mt
argv =    ["", 
            f"{MAXTEXT_CONFIGS_DIR}/base.yml", 
            f"load_parameters_path={MODEL_CHECKPOINT_PATH}/0/items",
         ]
model,mesh, config = mt.from_pretrained(argv, model_name=“Qwen3-0.8b”)

Tests

Please describe how you tested this change, and include any instructions and/or
commands to reproduce.

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