Fix Issue 57: Add support for L4 GPUs in Vertex jobs

[tripathiji1312]

This PR resolves issue #57 by adding support for L4 GPUs when launching jobs on Vertex AI.

Description
Currently, attempting to use an L4 GPU by specifying L4_24TH in xm.JobRequirements results in a KeyError: 'NVIDIA_TESLA_L4_24TH'. This is because the accelerator type was being constructed by prefixing the resource name with NVIDIA_TESLA_, which is not the correct identifier for L4 GPUs on Vertex AI. The correct accelerator type is NVIDIA_L4.

This change updates vertex.py to special-case the L4_24TH GPU type and use the correct NVIDIA_L4 accelerator name. A corresponding unit test has been added to vertex_test.py to verify the fix and prevent future regressions.

Changes
Modified vertex.py to correctly handle the L4 GPU accelerator type.
Added test_get_machine_spec_l4 to vertex_test.py to ensure L4 GPUs are correctly configured

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[hartikainen]

You might be interested in #58.