Refactor hardcoded container mount paths to use constants

Description

As introduced in PR #814, three new constants were added to SlurmCommandGenStrategy for container mount paths:

• CONTAINER_MOUNT_INSTALL = "/cloudai_install"
• CONTAINER_MOUNT_OUTPUT = "/cloudai_run_results"
• CONTAINER_MOUNT_HF_HOME = "/cloudai_install/huggingface"

These constants should be used consistently across the codebase instead of hardcoded strings.

Files with hardcoded paths

Source files:

• src/cloudai/systems/slurm/slurm_command_gen_strategy.py (line 309)
• src/cloudai/workloads/nemo_run/slurm_command_gen_strategy.py (line 124)
• src/cloudai/workloads/ai_dynamo/ai_dynamo.sh (lines 4-6, 17)
• src/cloudai/workloads/triton_inference/slurm_command_gen_strategy.py (line 147)

Test files:

• tests/test_single_sbatch_runner.py (lines 268-269, 277)
• tests/workloads/slurm_container/test_command_gen_strategy_slurm.py (lines 49-50, 67-68, 87-88)
• tests/workloads/nemo_run/test_command_gen_strategy_slurm.py (line 74)
• tests/systems/slurm/test_command_gen_strategy.py (lines 239-240, 258-259, 272-273)

Proposed refactoring

1. Replace hardcoded strings in source files with references to SlurmCommandGenStrategy.CONTAINER_MOUNT_* constants
2. Update test files to use the constants for consistency
3. Consider making these constants accessible in a more central location if needed by non-Slurm code

Context

• PR: https://github.com/NVIDIA/cloudai/pull/814
• Comment: https://github.com/NVIDIA/cloudai/pull/814#discussion_r2863706958