[training, test] feat: add unit tests for theoretical_memory_utils

[lonexreb]

Summary

`src/megatron/bridge/training/utils/theoretical_memory_utils.py` exports three public helpers that compute model memory footprint analytically from a `ConfigContainer`:

• `compute_weight_and_optimizer_memory`
• `compute_activation_memory`
• `report_theoretical_memory`

They do pure arithmetic on dataclass fields — no GPU, no model construction, no checkpoints — so they're a natural unit-test target that was previously uncovered.

What this PR adds

19 tests across four classes in `tests/unit_tests/training/utils/test_theoretical_memory_utils.py`:

`TestComputeWeightAndOptimizerMemory` (11)

• Returns positive float
• Non-distributed optimizer uses 18 bytes/param
• Distributed optimizer scales with `data_parallel_size` (6 + 12/dp)
• Tensor parallel reduces per-shard memory
• Separate (non-shared) embeddings increase param count
• Gated linear + SiLU adds MLP params (and the gated-linear flag alone, without SiLU, is correctly not treated as gated)
• More layers / more MoE experts increase memory
• GQA (`num_query_groups < num_attention_heads`) reduces attention params
• Verbose mode prints layer counts
• PP > 1 verbose mode logs "other shards"

`TestComputeActivationMemory` (7)

• Returns positive float
• Layer / TP / micro-batch sensitivity
• Virtual-PP interleaved schedule memory penalty
• Non-interleaved PP microbatch discount (`min(1, num_microbatches/pp)`)
• PP=1 output-layer / CE loss term executes
• Verbose mode prints interleaved penalty / microbatch info

`TestReportTheoreticalMemory` (3)

• Skips activation breakdown when `sequence_parallel=False`
• Prints full `weight=… activation=… total=…` when seq-parallel + selective recompute
• Skips activation breakdown when `recompute_granularity != "selective"`

`MegatronMIMOProvider` is patched out so the early-return MIMO check doesn't short-circuit normal configs.

`TestNumBytesInMegabyteConstant` (1)

• Sanity check on the public byte constant

Why

Follows the recently-merged unit-test pattern from #3650 (pg_utils) and #3695 (FLOPs calculator) — small, GPU-free, high-signal coverage of an analytic helper.

Test plan

• 19 new tests, all in `tests/unit_tests/` so they run under `Launch_Unit_Tests_Core` (no functional / GPU slot)
• Mocks use `SimpleNamespace` and `unittest.mock.patch` — no real model build
• No production code changed
• CI `Launch_Unit_Tests_Core` passes

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